Use these links to rapidly review the document
TABLE OF CONTENTS
ANNEX B CONTENTS

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of
the Securities Exchange Act of 1934 (Amendment No.          )

 

---

Table of Contents

          You are cordially invited to the Special Meeting of Stockholders of Aon Corporation, a Delaware corporation ("Aon Delaware"), to be held at the offices of Aon Corporation, located at the Aon Center, 200 East Randolph Street, Chicago, Illinois, on March 16, 2012, at 1:00 p.m., local time. Only stockholders of record at the close of business on February 6, 2012 are entitled to notice of and to vote at the special meeting. Details of the business to be presented at the special meeting can be found in the accompanying Notice of Special Meeting of Stockholders and proxy statement/prospectus.

          At the special meeting, you will be asked to approve the adoption of an Agreement and Plan of Merger and Reorganization (the "merger agreement") between Aon Delaware and a wholly-owned, indirect subsidiary of Aon Delaware, which is one of the initial steps of a proposed reorganization of the corporate structure of Aon Delaware and the group of companies it controls (the "reorganization"). If the merger agreement is approved, we would consummate a merger the result of which is that the place of incorporation of our parent company would change from Delaware to England and you would hold Class A Ordinary Shares ("Class A Ordinary Shares") of a newly formed public limited company named "Aon plc" or a similar name ("Aon UK"), incorporated under English law and subject to U.K. tax laws. Aon UK is currently a private limited company named "Aon Global Limited." The number of Class A Ordinary Shares that you will own will be the same as the number of shares of common stock of Aon Delaware that you own immediately prior to the completion of the merger, and your proportionate ownership and relative voting rights will remain unchanged, subject, in each case, to your receipt of cash in lieu of any fractional shares that you own. We currently anticipate that the merger will be completed in the second quarter of 2012, although the merger may be abandoned by our Board at any time, including after stockholder approval.

          After the completion of the merger, Aon UK and its subsidiaries will continue to conduct the same business operations as conducted by Aon Delaware before the transaction. We will submit an application to the New York Stock Exchange (the "NYSE") and expect that, immediately following the effective time of the merger, the Class A Ordinary Shares will be listed on the NYSE under the symbol "AON," the current symbol for Aon Delaware common stock. We have no current plans to list Aon UK's shares on any other securities exchange, including the London Stock Exchange.

          Upon completion of the reorganization, Aon UK will remain subject to the U.S. Securities and Exchange Commission reporting requirements, the mandates of the Sarbanes-Oxley Act and the applicable corporate governance rules of the NYSE, and Aon UK will continue to report consolidated financial results in U.S. dollars and in accordance with U.S. generally accepted accounting principles. Aon UK also must comply with any additional reporting requirements of English law.

          Generally, for U.S. federal income tax purposes (i) stockholders of Aon Delaware who are U.S. persons should recognize gain, if any (but not loss), as a result of the merger, and (ii) stockholders of Aon Delaware who are not U.S. persons will not be subject to U.S. federal income tax as a result of the merger.

          At the special meeting, you will also be asked to approve a proposal to implement a reduction of capital of Aon UK through a customary court-approved process so that it will have sufficient distributable reserves under English law to declare dividends, make distributions or repurchase shares following the completion of the merger.

          You will also be asked to approve, on an advisory and non-binding basis, compensation that may be payable to certain named executive officers of Aon Delaware in connection with the merger and their relocation to the U.K. (the "relocation compensation").

          Finally, you will be asked to vote on a proposal to adjourn the special meeting, if necessary, to solicit additional votes if there are not sufficient votes in favor of Proposal 1.

          As further explained in the accompanying proxy statement/prospectus, we currently have substantial operations in the U.K. Our Board of Directors expects that the reorganization and the relocation of many of our senior executive officers and other key decision makers to the U.K. would, among other anticipated benefits, enhance our focus on growth by positioning our resources more proximately to the geographic areas poised to experience the highest level of growth, enable our Risk Solutions business to better exploit key retail and reinsurance initiatives that are or will be substantially driven from the U.K., allow us to expand the penetration of our HR Solutions segment into Europe, leverage our development, retention and acquisition of professional talent and optimize our fiscal planning and capital allocation and reduce our global tax rate in a manner that provides us with increased financial flexibility to invest in our growth, product innovation, expansion and talent development strategies while delivering increased shareholder value.

          Our Board of Directors has determined that the reorganization and the merger agreement are advisable and in the best interests of Aon Delaware and our stockholders and, as such, has approved the reorganization and the merger agreement. The Board of Directors recommends that you vote "FOR" the proposal to adopt the merger agreement, "FOR" the proposal to approve the implementation of a reduction of capital of Aon UK so that it will have sufficient distributable reserves under English law to declare dividends, make distributions or repurchase shares following the completion of the merger, "FOR" the proposal, on an advisory and non-binding basis, to approve the relocation compensation described in Proposal 3 and "FOR" the proposal to adjourn the special meeting if there are not sufficient votes in favor of Proposal 1.

        Your vote is very important.    Whether or not you plan to attend the special meeting, please take the time to vote by completing and mailing the enclosed proxy card or voting instruction card or, if the option is available to you, by granting your proxy electronically over the Internet or by telephone.

          The reorganization cannot be completed unless the proposal to adopt the merger agreement is approved by the holders of at least two-thirds of Aon Delaware's outstanding shares entitled to vote at the special meeting. Adoption of the merger agreement, approval of the reduction of capital of Aon UK, and approval of the relocation compensation are subject to separate votes by the Aon Delaware stockholders, and approval of the reduction of capital of Aon UK and approval of the relocation compensation are not conditions to the completion of the merger. This proxy statement/prospectus contains important information about us, the merger agreement, the proposed merger and the special meeting. I encourage you to read carefully this proxy statement/prospectus before voting, including the section entitled "Risk Factors" beginning on page 24.

Sincerely,

Gregory C. Case
President and Chief Executive Officer
Aon Corporation

          Neither the U.S. Securities and Exchange Commission, any state securities regulator nor the U.K.'s Financial Services Authority (the "FSA") has approved or disapproved of the transactions described in this proxy statement/prospectus or the securities to be delivered in the merger or determined if this proxy statement/prospectus is truthful or complete. Any representation to the contrary is a criminal offense. For the avoidance of doubt, this proxy statement/prospectus is not intended to be and is not a prospectus or circular for purposes of the U.K. FSA's Prospectus Rules and Listing Rules.

          This proxy statement/prospectus is dated February 6, 2012, and is first being mailed to stockholders of Aon Delaware on or about February 10, 2012.

---

Table of Contents

AON CORPORATION
200 East Randolph Street
Chicago, IL 60601

NOTICE OF SPECIAL MEETING OF STOCKHOLDERS

TO BE HELD ON MARCH 16, 2012

        We will hold a special meeting of stockholders of Aon Corporation at the offices of Aon Corporation, located at the Aon Center, 200 East Randolph Street, Chicago, Illinois, on March 16, 2012, at 1:00 p.m., local time, for the following purposes:

        1.     To consider and vote upon a proposal to adopt the Agreement and Plan of Merger and Reorganization (the "merger agreement"), a copy of which is attached to the accompanying proxy statement/prospectus as Annex A, by and between Aon Corporation, a Delaware corporation ("Aon Delaware"), and Market Mergeco Inc., a newly formed Delaware corporation ("Aon Mergeco") and a wholly-owned subsidiary of Aon Holdings LLC, a newly formed Delaware limited liability company ("Aon Intermediate") and a wholly-owned subsidiary of Aon Delaware, pursuant to which Aon Mergeco will merge (the "merger") with and into Aon Delaware, with Aon Delaware surviving the merger as a wholly-owned subsidiary of Aon Intermediate. Aon Intermediate will become, in connection with the merger, a wholly-owned subsidiary of Aon Global Limited, a newly formed private limited company incorporated under English law which, prior to the effective time of the merger, will re-register as a public limited company named "Aon plc" or a similar name ("Aon UK"). Pursuant to the merger agreement, each issued and outstanding share of the common stock of Aon Delaware will be converted into the right to receive one Class A Ordinary Share of Aon UK, nominal value $0.01 per share. Aon UK, together with its subsidiaries, will own and continue to conduct our business in substantially the same manner as is currently being conducted by Aon Delaware and its subsidiaries.

        2.     To consider and vote upon a proposal to approve the implementation of a reduction of capital of Aon UK through a customary court-approved process so that it will have sufficient distributable reserves under English law to declare dividends, make distributions or repurchase shares following the completion of the merger.

        3.     To consider and vote upon a proposal to approve, on an advisory and non-binding basis, compensation that may be payable to certain named executive officers of Aon Delaware in connection with the merger and their relocation to the U.K. (the "relocation compensation").

        4.     To consider and vote upon a proposal to adjourn the special meeting, if necessary, to solicit additional proxies if there are not sufficient votes in favor of proposal 1.

        Only stockholders of record at the close of business on February 6, 2012, the record date for the special meeting, are entitled to notice of and to vote at the special meeting.

        The Aon Delaware board of directors recommends that you vote "FOR" the adoption of the merger agreement, "FOR" the approval of the reduction of capital of Aon UK, "FOR" the approval, on an advisory and non-binding basis, of the relocation compensation and "FOR" the adjournment of the special meeting, if necessary, to solicit additional proxies if there are not sufficient votes in favor of proposal 1.

        A list of stockholders eligible to vote at the special meeting will be available for inspection at the special meeting and at the executive offices of Aon Corporation during regular business hours for a period of no less than ten days prior to the special meeting.

---

Table of Contents

        Your vote is very important.    It is important that your shares be represented and voted whether or not you plan to attend the special meeting in person. Instructions regarding the different methods for voting your shares are provided under the section entitled "General Information About the Merger and the Special Meeting—Questions and Answers Relating to the Special Meeting" beginning on page 10.

February 6, 2012

---

Table of Contents

TABLE OF CONTENTS

i

---

Table of Contents

ii

---

Table of Contents

        In this proxy statement/prospectus, we refer to Aon Corporation, the Delaware corporation whose shares you currently own (together with its subsidiaries where applicable), as "Aon" or "Aon Delaware," or the "Company." Additionally, we sometimes refer to Aon Delaware as "we," "us," or "our." "You" refers to the stockholders of Aon Delaware. We refer to Aon Global Limited, a newly formed private limited company incorporated under English law (which, prior to the effective time of the merger, will re-register as a public limited company and be renamed "Aon plc" or a similar name) as "Aon UK." A reference in this proxy statement/prospectus to a Class A Ordinary Share means a Class A Ordinary Share of Aon UK, nominal value $0.01 per share. The number of Class A Ordinary Shares to be delivered to the stockholders of Aon Delaware is equal to the number of shares of common stock of Aon Delaware held by the stockholders of Aon Delaware immediately prior to the effective time of the merger described in this proxy statement/prospectus. In addition, unless the context indicates otherwise, when we refer to shares of Aon Delaware common stock issued and outstanding, we are excluding the treasury shares of Aon Delaware common stock owned by or for the benefit of Aon Delaware or its subsidiaries. References to the "special meeting" also refer to any adjournment or postponement of the special meeting, if necessary. Unless otherwise noted, all monetary amounts are stated in U.S. dollars.

        This proxy statement/prospectus incorporates important business and financial information about Aon Delaware from documents Aon Delaware has filed with the U.S. Securities and Exchange Commission, or the "SEC," that are not included in or delivered with this proxy statement/prospectus. See "Where You Can Find More Information."

        You can obtain the documents incorporated by reference into this proxy statement/prospectus by accessing the SEC website maintained at www.sec.gov. Aon Delaware will also provide you with copies of the documents incorporated by reference into this proxy statement/prospectus, without charge. Those documents are available to any stockholder, including any beneficial owner, upon request directed to Aon Corporation at 200 East Randolph Street, Chicago, Illinois 60601, Attention: Investor Relations, telephone: (312) 381-1000.

        To ensure timely delivery of such documents, any request should be made no later than five business days prior to the date of the special meeting. The exhibits to those documents will generally not be made available unless they are specifically incorporated by reference into this proxy statement/prospectus or are otherwise requested by you.

        In addition, you may obtain copies of some of this information by accessing Aon Delaware's website at www.aon.com under the heading "Investor Relations," and then under the link "SEC Filings." We are providing the information about how you can obtain certain documents that are incorporated by reference into this proxy statement/prospectus at these websites only for your convenience. We are not incorporating the contents of the websites of the SEC or Aon Delaware or any other entity into this proxy statement/prospectus.

iii

---

Table of Contents

PROXY STATEMENT FOR THE SPECIAL MEETING OF
STOCKHOLDERS OF AON CORPORATION

TO BE HELD ON MARCH 16, 2012

        This proxy statement/prospectus and a proxy card or voting instruction card are first being sent or distributed to stockholders of Aon on or about February 10, 2012. Our Board of Directors, or the "Board," is soliciting your proxy to vote your shares at the Special Meeting of Stockholders to be held at the offices of Aon Corporation, located at the Aon Center, 200 East Randolph Street, Chicago, Illinois, on March 16, 2012, at 1:00 p.m., local time, or the "special meeting," and any adjournments or postponements of the special meeting. Our Board is soliciting your proxy to give all stockholders of record the opportunity to vote on matters that will be presented at the special meeting and any adjournments or postponements of the special meeting. This proxy statement/prospectus provides information on these matters to assist you in voting your shares.

GENERAL INFORMATION ABOUT THE MERGER AND THE SPECIAL MEETING

        The following questions and answers are intended to address briefly some commonly asked questions regarding the proposed merger and the special meeting. These questions and answers may not address all questions that may be important to you. Please refer to the more detailed information contained elsewhere in this proxy statement/prospectus, its annexes and exhibits and the documents referred to or incorporated by reference in this proxy statement/prospectus. For instructions on obtaining the documents incorporated by reference, see "Where You Can Find More Information."

Why am I receiving this proxy statement/prospectus?

        Our Board has approved a reorganization of our corporate structure, which would include changing the jurisdiction of incorporation of our parent company from Delaware to England and relocating our principal executive offices and additional senior executive officers to the U.K. We would change the jurisdiction of incorporation of our parent company through a merger that will result in Aon UK becoming the parent company of the Aon group of companies and effectively will result in your owning shares in an English public limited company instead of shares in a Delaware corporation. The merger agreement requires stockholder approval, which is why we have called a special meeting of stockholders and sent you this proxy statement/prospectus.

Questions and Answers Relating to the Special Meeting Proposals

Who are the parties to the merger?

        The parties to the merger described in this proxy statement/prospectus are Aon Delaware and Market Mergeco Inc., or "Aon Mergeco," a newly formed Delaware corporation. Aon Mergeco is a wholly-owned subsidiary of Aon Holdings LLC, a newly formed Delaware limited liability company, or "Aon Intermediate." Aon Intermediate is a subsidiary of Aon Delaware.

What is the merger?

        Under the Agreement and Plan of Merger and Reorganization, or the "merger agreement," Aon Mergeco will merge into Aon Delaware (which we refer to as the "merger"), with Aon Delaware surviving the merger as a wholly-owned subsidiary of Aon Intermediate. Upon consummation of the merger, each issued and outstanding share of common stock of Aon Delaware will be converted into the right to receive one Class A Ordinary Share. Aon UK, together with its subsidiaries (including Aon Delaware), will own and continue to conduct our business in substantially the same manner as is currently being conducted by Aon Delaware and its subsidiaries.

1

---

Table of Contents

What are the major actions that have been performed or will be performed to effect the merger?

        Aon Delaware has taken or will take the actions listed below to effect the merger. Charts describing the ownership structure among Aon Delaware, Aon UK, Aon Intermediate and Aon Mergeco before and after the effective time of the merger are also set forth below.

Major Actions

•

Aon Delaware formed Aon UK as its wholly-owned subsidiary.



•

Aon Delaware deposited the Class A Ordinary Shares of Aon UK with EES Trustees International Limited and an affiliated nominee, or "EES". Aon Delaware and EES entered into a depositary agreement governing the deposit of the Class A Ordinary Shares with EES.



•

Aon Delaware formed Aon Intermediate as its wholly-owned subsidiary.



•

Aon Intermediate formed Aon Mergeco as its wholly-owned subsidiary.



•

Conditional upon the effectiveness of the registration statement of which this proxy statement/prospectus is a part, (i) Aon Delaware will contribute all of its interests in the Class A Ordinary Shares to Aon Intermediate, and (ii) Aon Intermediate will contribute all of its interests in the Class A Ordinary Shares to Aon Mergeco.



•

Conditional upon approval of the merger agreement by its stockholders, Aon Delaware will contribute all shares of Aon Intermediate to Aon UK.



•

Aon Mergeco and Aon Delaware will merge, and Aon Mergeco will cause the Class A Ordinary Shares to be delivered to the Aon Delaware stockholders for exchange pursuant to the merger.

Structure Chart Prior to and after the Effective Time of the Merger

        The following diagram depicts our organizational structure before and after the merger. The diagram does not depict any legal entities owned by Aon Delaware other than those related to the merger nor any aspects of the reorganization that are anticipated to take place after the effective time of the merger.

What will be Aon's corporate presence in the U.K.?

        In conjunction with the reorganization, we plan to expand our already substantial presence in the U.K., which will include relocating our principal executive offices to the U.K. Many of our senior

2

---

Table of Contents

executive officers and key decision makers are already in the U.K., and we expect that more will move to the U.K. We also intend to hold more of our regularly scheduled board meetings in the U.K.

What will I receive in the merger?

        The merger agreement provides that each share of Aon Delaware issued and outstanding immediately prior to the effective time of the merger will be converted into the right to receive one Class A Ordinary Share of Aon UK.

Will the merger dilute my economic interest?

        No. Immediately after the effective time of the merger, your relative economic ownership in Aon UK will remain the same as your relative economic ownership of Aon Delaware immediately before the merger, subject to your receipt of cash in lieu of any fractional shares that you hold.

Why does Aon want to engage in the merger?

        The merger is part of a reorganization of Aon's corporate structure, which includes the relocation of the place of incorporation of our parent company and our principal executive offices to the U.K., and which we expect will enable us to expand our long-standing presence in the U.K. while concurrently investing in our business globally.

        We expect that the reorganization of our corporate structure, which we refer to as the "reorganization," will strengthen our long-term strategy by:

•

Enhancing our focus on growth by positioning our resources more proximately to the geographic areas poised to experience the highest level of growth and which areas today derive a substantial portion of their risk coverage through the U.K. market;



•

Enabling our Risk Solutions business to deliver superior value to our clients and differentiate us from our competition via key retail initiatives that are or will be substantially driven from the U.K., such as our Global Risk Insight Platform, or "GRIP", Programs & Facilities and Broking Excellence and key reinsurance initiatives that are or will be substantially driven from the U.K., such as facultative programs and further expanding our Aon Re Analytics capabilities;



•

Allowing us to expand the penetration of our HR Solutions segment into Europe through London, the current base for much of our European consulting operations and growing our U.K. and Europe, the Middle East and Africa, or "EMEA", benefits administration and human resources business process outsourcing, or "HR BPO", business lines;



•

Leveraging our development, retention and acquisition of professional talent and optimizing its geographic deployment around the world in each of our primary and developing markets;



•

Optimizing our fiscal planning and capital allocation and reducing our global tax rate in a manner that provides us with increased financial flexibility to invest in our growth, Risk Solutions product innovation, HR Solutions expansion and talent development strategies while delivering increased shareholder value as follows:



•

Providing greater global access, as a result of the U.K.'s tax rules regarding the repatriation of earnings, to expected increases in future cash flow as Aon delivers on its long-term operating margin targets while certain required uses of cash decrease over the next several years as restructuring costs decline and contributions to our material pension plans that are closed to new entrants and, where statutorily permitted, frozen from continued benefit accruals, are reduced as those plans become more fully funded;



•

Enabling us to access approximately $300 million of excess capital held internationally on our balance sheet upon consummation of this reorganization;

3

---

Table of Contents

•

Increasing future cash flows through a significant reduction in our global tax rate over the long term due to changes in the geographic distribution of income, similar to the reduction achieved over the last five years, which would allow us to remain competitive with certain global competitors; and



•

Delivering increased shareholder value as we effectively allocate capital through share repurchases and dividends.

        We also expect that the reorganization will result in operational and administrative efficiencies over the long term and enhance our ability to further expand in the Europe, Middle East & Africa and Asia Pacific geographic regions, which represented approximately 27 percent of our consolidated revenue for the year ended December 31, 2010 on a pro forma basis assuming the acquisition of Hewitt on January 1, 2010. As we continue to grow our business internationally, we believe that relocating our principal executive offices to the U.K. will provide increased strategic flexibility and operational benefits. We thus believe the merger will allow us to compete more effectively on a global scale. See "Proposal 1—Approval of the Adoption of the Merger Agreement—Background and Reasons for the Merger."

Will the merger affect current or future operations?

        Our principal executive offices will be moved to the U.K., which we expect will create more operational efficiencies. However, the merger is not expected to have a material impact on how we conduct day-to-day operations in the United States below the senior executive level. We expect, for example, to increase the size of our Chicago and other U.S.-based workforce as a result of the reorganization. While the new corporate structure would not change our future operational plans to grow our business or diminish our focus on our U.S. business, it may improve our ability to expand within our Europe, Middle East & Africa and Asia Pacific geographic regions. The location of future operations will depend on the needs of the business. Please see "Proposal 1—Approval of the Adoption of the Merger Agreement—Background and Reasons for the Merger."

What actions are expected to be taken following the merger?

        Following the consummation of the merger, Aon UK, as Aon Delaware's ultimate parent company, expects to examine various potential transactions that could bring about a further restructuring of Aon Delaware and certain of its subsidiaries. We refer to these activities and transactions in this proxy statement/prospectus as "subsequent actions." Refer to "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" below for a description of certain risks associated with the reorganization.

When do you expect to complete the merger?

        If the proposal to adopt the merger agreement is approved by our stockholders at the special meeting, we anticipate that the merger will become effective as soon as practicable following stockholder approval. We currently anticipate that the merger will be completed in the second quarter of 2012, although the merger may be abandoned by our Board at any time, including after stockholder approval. Please see "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors."

Is the merger taxable to me?

        For U.S. federal income tax purposes, the conversion of each issued and outstanding share of the common stock of Aon Delaware into the right to receive a Class A Ordinary Share upon the consummation of the merger generally will be treated as an exchange of such share for the Class A Ordinary Share. Therefore, the U.S. federal income tax discussion contained herein refers to an "exchange" rather than a "conversion" in describing the U.S. federal income tax consequences of the merger.

4

---

Table of Contents

        Generally, for U.S. federal income tax purposes, a "U.S. holder," which is defined below in "Material Tax Considerations Relating to the Merger—U.S. Federal Income Tax Considerations—Material Tax Consequences to Stockholders," should recognize gain, if any, but not loss, on the receipt of Class A Ordinary Shares in exchange for Aon Delaware common stock in connection with the merger. A U.S. holder should generally recognize gain equal to the excess, if any, of the fair market value of the Class A Ordinary Shares received in the merger over the U.S. holder's adjusted tax basis in the shares of Aon Delaware common stock. Generally, this gain should be a capital gain. U.S. holders should not be permitted to recognize any loss realized on the exchange of their shares of Aon Delaware common stock in the merger. In determining the amount of gain recognized, each of the Aon Delaware shares exchanged should be treated as the subject of a separate exchange. Thus, if a U.S. holder exchanges some Aon Delaware shares on which gains are realized and other Aon Delaware shares on which losses are realized, the U.S. holder may not net the losses against the gains to determine the amount of gain recognized. In the case of a loss, the adjusted tax basis in each Class A Ordinary Share received by a U.S. holder should equal the adjusted tax basis of each share of Aon Delaware common stock exchanged therefor.

        Cash received by a U.S. holder in exchange for a fractional share of Aon Delaware common stock should be treated as having been received in redemption of such fractional share. Thus, gain or loss generally should be recognized by such U.S. holder in an amount equal to the difference between the amount of cash received and the U.S. holder's adjusted tax basis in such fractional share. A non-U.S. holder generally will not be subject to U.S. federal income tax on gain recognized, if any, on the exchange of Aon Delaware shares for Class A Ordinary Shares or on the receipt of cash in exchange for fractional shares of Aon Delaware common stock. Please see "Material Tax Considerations Relating to the Merger—U.S. Federal Income Tax Considerations—Material Tax Consequences to Stockholders."

        For U.K. tax purposes, stockholders of Aon Delaware who are residents of the U.S., and not residents of the U.K., will generally not be subject to U.K. taxation as a result of the merger. Generally, U.K. resident stockholders of Aon Delaware may realize a chargeable gain or an allowable loss for U.K. corporation tax or capital gains tax purposes (as the case may be) as a result of the merger, which will be treated as giving rise to a disposal of their stock in Aon Delaware. Generally, non-U.K. resident stockholders of Aon Delaware should not be subject to either U.K. corporation tax or capital gains tax as a result of the merger, unless they carry on a trade in the U.K. through a permanent establishment in the U.K. or through a branch or agency in the U.K. for the purposes of U.K. corporation tax or capital gains tax, respectively, and the stock is attributable to that permanent establishment, branch or agency. In general, you will be treated as acquiring the Class A Ordinary Shares received in the merger for consideration equal to the market value at the effective time of the merger of your shares of Aon Delaware common stock converted in the merger. Please see "Material Tax Considerations Relating to the Merger—U.K. Tax Considerations—Material Tax Consequences to Shareholders."

        For stockholders of Aon Delaware who are citizens or residents of, or otherwise subject to taxation in, a country other than the U.S. or the U.K., the tax treatment of the merger will depend on the applicable tax laws in such country.

        WE URGE YOU TO CONSULT YOUR OWN TAX ADVISOR PRIOR TO THE SPECIAL MEETING REGARDING THE PARTICULAR TAX CONSIDERATIONS OF THE MERGER FOR YOU.

Will Aon Delaware or Aon UK be taxed as a result of the merger?

        We believe that Aon Delaware and Aon UK should not incur any significant taxes in connection with consummation of the merger. Although changes in tax laws, treaties or regulations or the interpretation or enforcement of these tax laws, treaties or regulations could adversely affect the intended tax benefits of the merger to Aon UK and its subsidiaries or the tax treatment of the

5

---

Table of Contents

post-merger corporate structure of Aon UK, we do not believe that any of such changes would result in a material increase in taxes as compared to our current, pre-merger tax position.

Has the IRS or HMRC rendered a ruling on any aspects of the merger?

        No ruling has been requested from the U.S. Internal Revenue Service, or "IRS," in connection with the merger, and the IRS has adopted a policy of not issuing a ruling that a merger qualifies as non-taxable, unless there is a separate significant issue related to the merger. Furthermore, the IRS will not rule on whether the U.S. anti-inversion rules will apply to the merger. See "Material Tax Considerations Relating to the Merger—U.S. Federal Income Tax Considerations—The U.S. Anti-Inversion Rules."

        No ruling has been obtained from H.M. Revenue & Customs, or "HMRC," as to the U.K. corporation tax consequences of the merger. We have obtained a ruling from HMRC that, following the merger, the "temporary period exemption" from the U.K.'s controlled foreign company rules will apply such that, subject to certain conditions and limitations based on our facts and circumstances, Aon UK will not be subject to tax on the profits of any controlled company which is resident in a foreign jurisdiction under the CFC rules until 24 months after the end of the accounting period in which the merger occurs, subject to any changes of legislation. See "Material Tax Considerations Relating to the Merger—Material U.K. Tax Considerations—Material U.K. Tax Considerations Relating to and After the Merger—After the Merger—Controlled Foreign Companies". A ruling has also been obtained from HMRC regarding the U.K. stamp duty and stamp duty reserve tax ("SDRT") consequences of the merger. See "Material Tax Considerations Relating to the Merger—Material U.K. Tax Considerations—Material U.K. Tax Considerations Relating to and After the Merger—The Merger."

What types of information and reports will Aon UK make available following the merger?

        After the effective time of the merger, Aon UK will continue to prepare financial statements in accordance with U.S. Generally Accepted Accounting Principles, or "U.S. GAAP," and report in U.S. dollars, and will continue to file reports under the U.S. Securities Exchange Act of 1934, as amended, or the "Exchange Act," including reports on Forms 10-K, 10-Q and 8-K with the SEC, as we currently do. Aon UK also must comply with additional reporting requirements of English law.

        For so long as Aon UK has a class of securities listed on the New York Stock Exchange, or "NYSE," Aon UK will be subject to rules regarding proxy solicitations and tender offers and the corporate governance requirements of the NYSE, the Exchange Act and the U.S. Sarbanes-Oxley Act of 2002 including, for example, independence requirements for audit committee composition, annual certification requirements and auditor independence rules, unless certain circumstances change. Aon UK will be required to disclose any significant ways in which its corporate governance practices differ from those followed by U.S. domestic companies under the NYSE's listing standards. To the extent possible under English law, Aon UK's corporate governance practices are expected to be comparable to those of Aon Delaware. Please see "Proposal 1—Approval of the Adoption of the Merger Agreement—Stock Exchange Listing," "Comparison of Rights of Stockholders/Shareholders," and "Proposal 1—Approval of the Adoption of the Merger Agreement—Effect of the Reorganization on Potential Future Status as a Foreign Private Issuer."

What must I do to obtain ownership of Class A Ordinary Shares that I become entitled to receive as a result of the merger?

        If you hold Aon Delaware share certificates, you will be sent a letter of transmittal, which is to be used to surrender your Aon Delaware share certificates and to request that Class A Ordinary Shares be delivered to you. The letter of transmittal will contain instructions explaining the procedure for surrendering Aon Delaware share certificates and requesting Class A Ordinary Shares. YOU SHOULD NOT RETURN SHARE CERTIFICATES WITH THE ENCLOSED PROXY CARD.

6

---

Table of Contents

        If you hold shares as a record holder in direct registration form (under the Direct Registration System) without a share certificate you will not be required to take any action. Your Class A Ordinary Shares will be issued in accordance with the arrangements described below.

        Beneficial holders of shares held in "street name" through a bank, broker or other financial institution will not be required to take any action. Your ownership of Class A Ordinary Shares will be recorded in book entry form by your broker without the need for any additional action on your part.

        All shares delivered in connection with the merger will initially be delivered in a manner that will allow those shares to be transferred through the facilities of The Depository Trust Company, or "DTC". As a result, if you hold share certificates representing Aon Delaware shares or if you otherwise hold Aon Delaware shares as a record holder of uncertificated shares, your shares will initially be delivered to Computershare Trust Company, N.A., an affiliate, or another bank or trust company, as custodian for you, or the "Custodian", and held in its account at DTC. No former record holder of Aon Delaware whose Class A Ordinary Shares are delivered to the Custodian will be required to pay any fees to the Custodian for the initial 90 day period. Any such holder who wishes to transfer its shares from the custody of the Custodian to another bank or broker or to receive certificated Class A Ordinary Shares will not be charged any fees to do so by the Custodian or Aon UK. Following the effective time, former record holders of Aon Delaware shares will also be provided with terms and conditions governing their custody account. Shareholders who wish the Custodian to continue holding their Class A Ordinary Shares must provide the Custodian with "know your customer" information and other customary documentation and must agree to the terms and conditions. A failure to provide such information and agreement within 90 days following the effective time will result in such holders no longer holding their shares through the Custodian and instead holding shares as record holders of certificated shares of Aon UK. Certain non-U.S. shareholders will not be eligible to hold their shares through the Custodian after the initial 90 day period. Any such shareholders who do not inform the Custodian of another bank or broker to whom their shares should be transferred prior to the end of the initial 90 day period will be issued shares in certificated form. You should particularly note that subsequent transfers of Class A Ordinary shares may attract stamp duty and SDRT under U.K. law. For more information, see "Material Tax Considerations Relating to the Merger—U.K. Tax Considerations—Material U.K. Tax Considerations Relating to and After the Merger—Stamp Duty and SDRT." As a result, each former record holder is strongly encouraged to provide the documents and information requested by the Custodian in a timely manner, so the shares may continue to be held within the facilities of DTC.

        For more information, see "Proposal 1—Approval of the Adoption of the Merger Agreement—Ownership in Aon UK; Conversion of Shares; Delivery of Shares to Former Record Holders."

        It is expected that, prior to the effective time of the merger, Computershare Inc. and/or one of its affiliates will be appointed as the exchange agent for the merger, or the "exchange agent." Aon UK's current share registrar is Computershare Investor Services PLC, P.O. Box 82, The Pavilions, Bridgwater Road, Bristol BS13 8AE, England, which will continue to serve as the U.K. share registrar for its Class A Ordinary Shares after the effective time of the merger. It is also expected that prior to the effective time of the merger, Computershare Trust Company, N.A., 250 Royall Street, Canton, Massachusetts, 02021 will be appointed to act as Aon UK's U.S. transfer agent as required under the rules of the New York Stock Exchange.

If I hold share certificates representing Aon Delaware shares, what will be my rights in relation to Aon UK before I exchange my certificates for Class A Ordinary Shares?

        From and after the effective time of the merger and pursuant to the terms of the merger agreement, Class A Ordinary Shares for stockholders who hold share certificates in Aon Delaware will initially be held in "street name" through the Custodian and, for so long as such shares are held by the Custodian, will be required to exercise their rights through the Custodian. Until Aon Delaware stock certificates are surrendered for exchange, any dividends or other distributions declared by Aon UK

7

---

Table of Contents

after the effective time of the merger with respect to Class A Ordinary Shares delivered to former Aon Delaware stockholders, other than any Class A Ordinary Shares held by Aon Delaware or any other subsidiary of Aon UK, will accrue but will not be paid to the Custodian, as custodian for such holder, and holders of such shares will not have voting rights with respect to Aon UK. See "Proposal 1—Approval of the Adoption of the Merger Agreement—Ownership in Aon UK; Conversion of Shares; Delivery of Shares to Former Record Holders" and "Proposal 1—Approval of the Adoption of the Merger Agreement—Dividends and Distributions; Withholdings; Termination of Exchange Fund; No Liability." It is expected that Computershare Inc. and/or one or more of its affiliates will serve as exchange agent for the exchange of Aon Delaware certificates for Class A Ordinary Shares.

What happens to Aon Delaware stock options, restricted stock units and other equity-based awards at the effective time of the merger?

        As of the effective time of the merger, Aon UK will adopt and assume, and become the plan sponsor of, the Aon Stock Incentive Plan, the Amended and Restated Global Stock and Incentive Plan of Hewitt Associates, Inc., the Aon Corporation 2011 Incentive Plan, and each change-in-control agreement with a key employee and under the Aon Corporation Executive Special Severance Plan, each as may be amended from time to time. With respect to all other existing plan and arrangements, Aon Delaware will remain the plan sponsor. With respect to all equity incentive plans or arrangements, however, Class A Ordinary Shares, rights to Class A Ordinary Shares or benefits or amounts based on Class A Ordinary Shares will thereafter be granted or issued under each equity incentive plan instead of shares of Aon Delaware currently being granted or issued under such plans. Future awards would be subject to, and governed by, the terms of our equity incentive plans, as assumed by Aon UK if applicable, and any agreements entered into pursuant thereto. Please see "Proposal 1—Approval of the Adoption of the Merger Agreement—Stock Compensation and Benefit Plans and Programs" for more information, including a description of the tax withholding and regulatory consequences of the merger.

        At the effective time of the merger, all outstanding options to purchase shares of Aon Delaware common stock and all outstanding awards of restricted stock units and other equity-based awards granted to employees and directors by the Company under our equity incentive plans prior to the effective time of the merger will entitle the holder to purchase or receive, or receive benefits or amounts based on, as applicable, an equal number of Class A Ordinary Shares. All of such equity-based awards will generally be subject to the same terms and conditions as were applicable to such awards granted or issued by Aon Delaware immediately prior to the effective time of the merger.

Can I trade my Aon Delaware shares before the merger is completed?

        Yes. Aon Delaware common stock will continue trading on the NYSE through the last trading day prior to the date of completion of the merger.

After the merger, where can I trade my Class A Ordinary Shares?

        It is a condition to the completion of the merger that the Class A Ordinary Shares will be authorized for listing on the NYSE. We will submit an application to the NYSE and expect that, immediately following the effective time of the merger, the Class A Ordinary Shares will be listed on the NYSE under the symbol "AON," the same symbol under which your Aon Delaware common stock is currently listed. We have no current plans to list Aon UK's shares on any other securities exchange, including the London Stock Exchange.

Why am I being asked to approve a reduction of capital of Aon UK?

        Under English law, Aon UK will only be able to declare dividends, make distributions or repurchase shares out of "distributable reserves." Immediately after the merger, as a newly formed public limited company, Aon UK will not have any distributable reserves. In order to have sufficient distributable reserves to continue paying quarterly dividends and to repurchase shares on the

8

---

Table of Contents

anticipated schedule for the foreseeable future, Aon UK will seek to have an amount approximately equal to the sum of Aon Delaware's retained earnings and additional paid-in capital less Aon Delaware's outstanding indebtedness as of a date on or around the closing created as distributable reserves following a reduction of capital of Aon UK implemented through a customary and conventional one-time court proceeding. Please see "Risk Factors—English law will require that we meet certain additional financial requirements before we declare dividends and repurchase shares following the merger", "Proposal 1—Approval of the Adoption of the Merger Agreement—Dividends", and "Proposal 2—Implementation of a Reduction of Capital of Aon UK" for more information. In particular, the current shareholder of Aon UK (which is Aon Delaware) has passed a resolution to reduce the capital of Aon UK to allow the creation of distributable reserves following the merger. If the proposal to reduce Aon UK's capital is approved by Aon Delaware's stockholders as set forth in Proposal 2 and the merger is completed, we will seek to obtain the approval of the English Companies Court through a customary process, which is required for the creation of distributable reserves to be effective, as soon as practicable following the transaction. The approval of the English Companies Court is expected to be received within four weeks after the completion of the merger.

What will happen if stockholders do not approve the reduction of capital of Aon UK?

        The approval by Aon Delaware's stockholders of the reduction of capital of Aon UK through a customary court-approved process is not a condition to the completion of the merger. If stockholders do not approve the proposal to approve an implementation of a reduction of capital of Aon UK through a court-approved process, Aon UK would achieve the reduction in capital through other methods to allow it to declare dividends, make distributions or repurchase shares following the completion of the merger. These efforts may include certain customary intra-group reorganizations, which are established and often-used alternatives for the creation of distributable reserves in a U.K. public limited company, but which we believe to be less advantageous than the proposed court-approved reduction in capital.

Why am I being asked to cast an advisory, non-binding vote to approve compensation that certain named executive officers of Aon Delaware may receive in connection with the merger?

        The Securities and Exchange Commission has adopted rules that require public companies to seek an advisory, non-binding vote with respect to certain payments received by named executive officers in connection with mergers and other corporate transactions. Because certain compensation that has been awarded by Aon Delaware to certain of its named executive officers in connection with the merger and their relocation to the U.K., or the "relocation compensation", may be deemed to be subject to these rules, we are holding an advisory vote regarding the relocation compensation.

Under what arrangements would relocation compensation be payable?

        Such compensation would be payable under international assignment letters entered into by Aon Delaware and the relocating executive officers. For more information, see "Proposal 1—Approval of the Adoption of the Merger Agreement—Interests of Directors and Executive Officers in the Merger".

What will happen if stockholders do not approve the relocation compensation at the special meeting?

        Approval of the relocation compensation that may be payable to certain of Aon Delaware's named executive officers in connection with the merger and their relocation to the U.K. is not a condition to completion of the merger. The vote with respect to such compensation is an advisory vote and will not be binding on Aon Delaware. Therefore, if the merger agreement is adopted by the stockholders and the merger is completed, the relocation compensation payable to certain of Aon Delaware's named executive officers will still be paid regardless of the outcome of this advisory vote.

9

---

Table of Contents

What is the voting requirement to approve the proposal to adopt the merger agreement? What is the voting requirement to approve the other proposals?

        For the merger to be able to proceed, at least two-thirds of the outstanding shares of common stock of Aon Delaware entitled to vote at the special meeting must be voted in the affirmative for the adoption of the merger agreement, as set forth in Proposal 1. Each of the other proposals will be approved if it receives the affirmative vote of the holders of a majority of the shares of common stock of Aon Delaware present in person or represented by proxy at the special meeting and entitled to vote thereon, whether or not a quorum is present.

What is the Board's voting recommendation?

        Our Board recommends that you vote your shares as follows:

        Proposal 1—FOR the adoption of the merger agreement;

        Proposal 2—FOR the approval of the implementation of a reduction of capital of Aon UK through a customary court-approved process so that it will have sufficient distributable reserves under U.K. law;

        Proposal 3—FOR the approval of the relocation compensation; and

        Proposal 4—FOR the approval of any adjournments of the special meeting to a later date to solicit additional proxies if there are insufficient votes at the time of the special meeting to approve the adoption of the merger agreement.

Questions and Answers Relating to the Special Meeting

When and where is the special meeting?

        Our special meeting will take place on March 16, 2012, at 1:00 p.m., local time, at the offices of Aon Corporation, located at the Aon Center, 200 East Randolph Street, Chicago, Illinois.

Who is entitled to notice of and to vote at the special meeting?

        Only stockholders of record on the record date (i.e., February 6, 2012) are entitled to notice of, and to vote at, the special meeting. As of the record date, our directors and executive officers and their affiliates beneficially owned, in the aggregate, approximately 1,049,455 of such shares, representing beneficial ownership of less than 0.32 percent of the outstanding shares of Aon Delaware common stock as of that date, and these shares are included in the number of shares entitled to vote at the special meeting.

Who may attend the meeting?

        Only Aon Delaware stockholders on the record date may attend the special meeting. If you are a stockholder of record, you will need to present the proxy card that you received, together with a form of personal photo identification, in order to be admitted into the meeting. If you hold your shares in "street name," you will need to provide proof of ownership, such as a recent account statement or letter from your bank, broker or other nominee, along with a form of personal photo identification. No cameras, recording equipment, electronic devices, large bags, briefcases or packages will be permitted into the meeting or adjacent areas. All other items may be subject to search.

What is this proxy statement/prospectus?

        This document serves as the proxy statement of Aon Delaware in connection with the solicitation of proxies to obtain votes on a proposal to adopt the merger agreement and the other proposals described herein. This document is also a prospectus of Aon UK for purposes of the U.S. Securities Act of 1933, as amended, or the "Securities Act," in connection with the issuance of Class A Ordinary

10

---

Table of Contents

Shares. For the avoidance of doubt, however, this proxy statement/prospectus is not intended to be and is not a prospectus or circular for purposes of the U.K. FSA's Prospectus Rules and Listing Rules.

We encourage you to read this proxy statement/prospectus carefully.

What is being delivered with the proxy statement/prospectus?

        These proxy materials include:

•

The notice of special meeting and proxy statement/prospectus for the special meeting; and



•

The accompanying proxy card (or voting instruction card from your broker or other intermediary) to register your vote on the proposal to adopt the merger agreement and the other proposals described herein.

Why am I receiving paper copies of these proxy materials when previously I received only a "Notice of Internet Availability of Proxy Materials" for the Company's annual stockholders' meeting?

        Under SEC rules, we are required to distribute paper copies of these proxy materials because of the subject matter of the business to be conducted at the special meeting.

Who is soliciting my proxy?

        Proxies are being solicited by the Board of Aon Delaware.

Who is paying for the cost of this proxy solicitation?

        We are paying the costs of soliciting proxies. Upon request, we will reimburse brokers, banks, trusts and other nominees for reasonable expenses incurred by them in forwarding the proxy materials to beneficial owners of shares of our common stock.

        In addition to soliciting proxies by mail and over the Internet, our Board, our officers and employees, or our transfer agent, may solicit proxies on our behalf, personally or by telephone, and we have engaged a proxy solicitor to solicit proxies on our behalf by telephone and by other means. We expect the cost of our private proxy solicitor to be approximately $20,000. We will also solicit proxies by e-mail from stockholders who are employees or who previously requested proxy materials be sent electronically. Arrangements also may be made with brokerage houses and other custodians, nominees and fiduciaries for the forwarding of solicitation materials to the beneficial owners of Aon Delaware shares held by those persons, and Aon Delaware will reimburse such persons for reasonable expenses incurred by them in connection with the forwarding of solicitation materials.

        Stockholders that vote through the Internet are advised that there may be costs associated with electronic access, such as usage charges from Internet access providers and telephone companies that will be borne by the stockholder in order for the stockholder to connect to or access the Internet site.

Whom should I contact with questions?

        We have retained Georgeson as proxy solicitor in connection with the special meeting. Our stockholders may contact our proxy soliciting agent at:

199 Water Street, 26^th Floor
New York, NY 10038

Banks and Brokers may call (212) 440-9800
All others may call toll-free at (866) 647-8869

Where are Aon Delaware's principal executive offices located and what is the main telephone number?

        Aon Delaware's principal executive offices are located at 200 East Randolph Street, Chicago, Illinois 60601 and the main telephone number is (312) 381-1000.

11

---

Table of Contents

What is the quorum requirement for the special meeting?

        For purposes of the special meeting, the holders of at least a majority of the shares of our common stock issued and outstanding and entitled to vote at the special meeting, present in person or represented by proxy, will constitute a quorum. For purposes of determining whether there is a quorum, abstentions and broker "non-votes," described below under "What happens if I do not give specific voting instructions? How are broker non-votes treated?" and "How are abstentions treated?", are treated as shares that are present.

What is the difference between holding shares as a stockholder of record and as a beneficial owner of shares held in street name?

        Stockholders of Record.    If your shares are registered directly in your name with our transfer agent, you are a "stockholder of record" with respect to those shares, and this proxy statement/prospectus was sent directly to you by us. As a record holder, you vote your shares directly by following the instructions set forth in the enclosed proxy card.

        Beneficial Owners of Shares Held in Street Name.    If your shares are held in the name of a broker, bank, trust or other nominee as a custodian or in the general account of the broker or other organization, you are a "street name" holder. Consequently, this proxy statement/prospectus was forwarded to you by that organization. The organization holding your account is considered the stockholder of record for purposes of voting at the special meeting. As a beneficial owner, you have the right to direct that organization on how to vote the shares held in your account by executing a voting instruction card. You should receive information regarding voting instructions directly from that organization.

If I am a stockholder of Aon Delaware, how do I vote?

        Stockholders of Record.    If you are a "stockholder of record," you have several choices. You can vote your shares by following the specific instructions provided on the proxy card:

•

via the Internet at www.envisionreports.com/Aon;



•

over the telephone by calling (800) 562-VOTE (8683); or



•

by mailing in the proxy card.

        Beneficial Owners of Shares Held in Street Name.    If you hold your shares in "street name," your broker, bank, trust or other nominee will arrange to provide materials and instructions for voting your shares.

        Beneficial Owners of Shares Held in the Aon Savings Plan.    If you are a current or former Aon Delaware employee who holds shares in the Aon Savings Plan, you will receive voting instructions from the trustee of the plan for shares allocated to your account. If you fail to give voting instructions to the trustee, your shares will be voted by the trustee in the same proportion as shares held by the trustee for which voting instructions were received. To allow sufficient time for voting by the trustee and administrator of the Aon Savings Plan, your voting instructions for shares held in the plan must be received by 5:00 p.m. Eastern Daylight Time on March 13, 2012.

What happens if I do not give specific voting instructions? How are broker non-votes treated?

        Stockholders of Record.    All properly executed proxy cards received by the Board from stockholders of record that do not specify how stock should be voted will be voted "FOR" the proposal to adopt the merger agreement and each of the other proposals. With respect to any other matter that may properly be brought before the special meeting, the shares shall be voted in accordance with the judgment of the person or persons named on the proxy card.

12

---

Table of Contents

        Beneficial Owners of Shares Held in Street Name.    If you are a beneficial owner of shares held in street name and do not provide the organization that holds your shares with specific voting instructions, under the rules of various national and regional securities exchanges, the organization that holds your shares may generally vote on "routine" matters but cannot vote on "non-routine" matters. If the organization that holds your shares does not receive instructions from you on how to vote your shares on a non-routine matter or otherwise elects not to vote your shares in the absence of your direction on a routine matter, the organization that holds your shares will inform our Inspector of Election that it does not have the authority (or otherwise declines) to vote on such matter with respect to your shares. This is generally referred to as a "broker non-vote."

        A vote on the proposal to adopt the merger agreement is regarded as a non-routine matter and failure to direct your broker on how to vote on such a proposal has the same effect as a vote AGAINST it. A broker non-vote with respect to any of the other proposals will have no effect. We encourage you to provide voting instructions to the organization that holds your shares by carefully following the instructions provided in the proxy card or voting instruction card.

Can I vote my shares in person at the special meeting?

        Stockholders of Record.    If you are a "stockholder of record," you may vote your shares in person at the special meeting.

        Beneficial Owners of Shares Held in Street Name.    If you hold your shares in "street name," you must obtain a proxy from your broker, bank, trust or other nominee giving you the right to vote the shares at the special meeting.

How are abstentions treated?

        In determining the number of votes cast for a proposal, shares abstaining from voting or not voted on a matter will be counted for quorum purposes. However, shares abstaining from voting or not voted with respect to any of the proposals will have the same effect as a vote AGAINST it.

May I change my vote after I have voted?

        You may revoke your proxy or otherwise change your vote by doing one of the following:

•

by sending a written notice of revocation to our Secretary that must be received prior to the special meeting, stating that you revoke your proxy;



•

by signing and submitting a later-dated proxy card that must be received prior to the special meeting in accordance with the instructions included in the proxy card; or



•

by attending the special meeting and voting your shares in person.

        If you voted electronically or by phone, you can also change your vote before the special meeting. Follow the same voting process and your original vote will be superseded.

        If you are a beneficial owner of our shares and a broker or other nominee holds your shares, you can revoke your proxy or otherwise change your vote by following the instructions provided by your broker or other nominee.

13

---

Table of Contents

14

---

Table of Contents

15

---

Table of Contents

16

---

Table of Contents

17

---

Table of Contents

18

---

Table of Contents

19

---

Table of Contents

20

---

Table of Contents

21

---

Table of Contents

22

---

Table of Contents

23

---

Table of Contents

RISK FACTORS

        In considering whether to vote in favor of the proposals contained in this proxy statement/prospectus, you should consider carefully the following risks or investment considerations, in addition to the other information in this proxy statement/prospectus, including the matters addressed in "Cautionary Statement Concerning Forward-Looking Statements" beginning on page 34. You should consider these risks when deciding how to vote. In addition, you should also review carefully the risk factors discussed in Part I, Item 1A "Risk Factors" of our Annual Report on Form 10-K for the annual period ended December 31, 2010 and any risk factors included in our subsequent reports filed pursuant to the Securities Exchange Act of 1934, as amended, as they discuss risks affecting our business generally that could also cause our actual plans or results to differ materially from those included in the forward-looking statements contained or incorporated by reference herein.

Some Aon Delaware stockholders will recognize a taxable gain as a result of the merger and, in some cases, will not be able to recognize any losses realized.

        For U.S. federal income tax purposes, the conversion of each issued and outstanding share of the common stock of Aon Delaware into the right to receive a Class A Ordinary Share upon the consummation of the merger generally will be treated as an exchange of such share for the Class A Ordinary Share. Therefore, the U.S. federal income tax discussion contained herein refers to an "exchange" rather than a "conversion" in describing the U.S. federal income tax consequences of the merger.

        Generally, for U.S. federal income tax purposes, a U.S. holder, which is defined below in "Material Tax Considerations Relating to the Merger—U.S. Federal Income Tax Considerations—Material Tax Consequences to Stockholders," should recognize gain, if any, but not loss, on the receipt of Class A Ordinary Shares in exchange for Aon Delaware common stock in connection with the merger. A U.S. holder should generally recognize gain equal to the excess, if any, of the fair market value of the Class A Ordinary Shares received in the merger over the U.S. holder's adjusted tax basis in the shares of Aon Delaware common stock exchanged therefor. Generally, this gain should be capital gain. U.S. holders should not be permitted to recognize any loss realized on the exchange of their shares of Aon Delaware common stock in the merger. In determining the amount of gain recognized, each of the Aon Delaware shares exchanged should be treated as the subject of a separate exchange. Thus, if a U.S. holder exchanges some Aon Delaware shares on which gains are recognized and other Aon Delaware shares on which losses are realized, the U.S. holder may not net the losses against the gains to determine the amount of gain recognized. In the case of a loss, the adjusted tax basis in each Class A Ordinary Share received by a U.S. holder should equal the adjusted tax basis of each share of Aon Delaware common stock exchanged therefor. In addition, the holding period for any Class A Ordinary Shares received by U.S. holders should include the holding period of the Aon Delaware shares exchanged therefor.

        Cash received by a U.S. holder in exchange for a fractional share of Aon Delaware common stock should be treated as having been received in redemption of such fractional share. Thus, gain or loss generally should be recognized by such U.S. holder in an amount equal to the difference between the amount of cash received and the U.S. holder's adjusted tax basis in such fractional share. A non-U.S. holder generally will not be subject to U.S. federal income tax on gain realized, if any, on the exchange of Aon Delaware shares for Class A Ordinary Shares or on the receipt of cash in exchange for fractional shares of Aon Delaware common stock. Please see "Material Tax Considerations Relating to the Merger—U.S. Federal Income Tax Considerations—Material Tax Consequences to Stockholders."

        Generally, U.K. resident stockholders of Aon Delaware may realize a chargeable gain or an allowable loss for U.K. corporation tax or capital gains tax purposes (as the case may be) as a result of the merger, which will be treated as giving rise to a disposal of their stock in Aon Delaware. Generally,

24

---

Table of Contents

non-U.K. resident stockholders of Aon Delaware should not be subject to either U.K. corporation tax or capital gains tax as a result of the merger, unless they carry on a trade in the U.K. through a permanent establishment in the U.K. or through a branch or agency in the U.K. for the purposes of U.K. corporation tax or capital gains tax respectively, and the stock is attributable to that permanent establishment, branch or agency. In general, you will be treated as acquiring the Class A Ordinary Shares received in the merger for a consideration equal to the market value at the effective time of the merger of your shares of Aon Delaware common stock converted in the merger. Please see "Material Tax Considerations Relating to the Merger—U.K. Tax Considerations—Material Tax Consequences to Shareholders."

        For stockholders of Aon Delaware who are citizens or residents of, or otherwise subject to taxation in, a country other than the U.S. or the U.K., the tax treatment of the merger will depend on the applicable tax laws in such country.

        WE URGE YOU TO CONSULT YOUR OWN TAX ADVISOR PRIOR TO THE SPECIAL MEETING REGARDING THE PARTICULAR TAX CONSIDERATIONS OF THE MERGER APPLICABLE TO YOU.

The expected benefits of the reorganization may not be realized.

        There can be no assurance that all of the goals of the reorganization will be achievable, particularly as the achievement of the benefits are in many important respects subject to factors that we do not control. These factors would include such things as the reactions of third parties with whom we enter into contracts and do business and the reactions of investors, analysts and U.K. and U.S. taxing authorities.

        While we expect that, as a result of the reorganization, we will benefit from the U.K. dividend exemption system for certain non-U.K. source dividends repatriated to the U.K. thereby increasing our financial flexibility, we cannot be assured that the benefits we expect will be received. In particular, U.K. or U.S. tax authorities may challenge our application and/or interpretation of relevant tax laws, regulations or treaties, valuations and methodologies or other supporting documentation, and, if they are successful in doing so, we may not experience the level of benefits we anticipate; or, we may be subject to adverse tax consequences. Even if we are successful in maintaining our positions, we may incur significant expense in contesting positions asserted or claims made by tax authorities, which may reduce the anticipated level of benefits.

        Our effective tax rates and the benefits described herein are also subject to a variety of other factors, many of which are beyond our ability to control, such as changes in the rate of economic growth in the U.K. and the U.S., the financial performance of our business in various jurisdictions, currency exchange rate fluctuations (especially as between the British pound and the U.S. dollar), and significant changes in trade, monetary or fiscal policies of the U.K. or the U.S., including changes in interest rates. The impact of these factors, individually and in the aggregate, is difficult to predict, in part because the occurrence of the events or circumstances described in such factors may be (and, in fact, often seem to be) interrelated, and the impact to us of the occurrence of any one of these events or circumstances could be compounded or, alternatively, reduced, offset, or more than offset, by the occurrence of one or more of the other events or circumstances described in such factors.

Aon UK may be treated as a U.S. corporation for U.S. federal tax purposes following the merger.

        Generally for U.S. federal income tax purposes, a corporation is considered a tax resident in the place of its incorporation. Because Aon UK is incorporated under U.K. law, it should be a U.K. corporation and a U.K. tax resident under these general rules. However, Section 7874 of the U.S. Internal Revenue Code of 1986, as amended (which we refer to as the "Code") generally provides that a corporation organized outside the U.S. which acquires substantially all of the assets of a corporation

25

---

Table of Contents

organized in the U.S. will be treated as a U.S. corporation (and, therefore, a U.S. tax resident) for U.S. federal tax purposes if former shareholders of the acquired U.S. corporation own at least 80 percent (of either the voting power or the value) of the stock of the acquiring foreign corporation after the acquisition and the expanded affiliated group does not have "substantial business activities" in the country in which the acquiring foreign corporation is organized. Pursuant to the merger, Aon UK will acquire directly or indirectly all of Aon Delaware's assets, and Aon Delaware stockholders will hold 100 percent of Aon UK by virtue of their stock ownership of Aon Delaware. As a result, the Aon Delaware expanded affiliated group and, following the merger, the Aon UK expanded affiliated group must have substantial business activities in the U.K. in order for Aon UK not to be treated as a U.S. corporation for U.S. federal tax purposes under Section 7874. There is no "safe harbor" or other guidance that confirms when an expanded affiliated group's business activities in a country of incorporation are deemed to be substantial. Therefore, it is possible that the IRS would interpret the Section 7874 "anti-inversion" rules so as to treat Aon UK as a U.S. corporation after the consummation of the merger and that such an IRS position would be sustained in litigation. Moreover, the United States Congress, the IRS, the United Kingdom Parliament or HMRC may enact new statutory or regulatory provisions that could adversely affect Aon UK's status as a non-U.S. corporation or otherwise adversely affect Aon UK's anticipated global tax position following the merger and any subsequent actions. Retroactive statutory or regulatory actions have occurred in the past, and there can be no assurance that any such provisions, if enacted or promulgated, would not have retroactive application to Aon UK, the merger or any subsequent actions.

        Although we believe Aon UK should not be treated as a U.S. corporation for U.S. federal tax purposes under Section 7874, there is no certainty that the IRS will not assert a contrary position, in which case, we could become involved in a tax controversy with the IRS regarding possible additional U.S. tax liability. If we are unsuccessful in resolving any such tax controversy in our favor, we would likely not realize the tax savings we expect to achieve through reorganization. Please see "Material Tax Considerations Relating to the Merger—U.S. Federal Income Tax Considerations—The U.S. Anti-Inversion Rules."

HMRC may disagree with our conclusions on the U.K. tax treatment of the merger, or relevant U.K. legislation may be subject to change.

        Based on current U.K. corporation tax law and the current U.K.-U.S. income tax treaty, as amended, and any amendments to either current U.K. corporation tax law or such treaty announced prior to the date hereof, we expect that the merger will not result in any material U.K. corporation tax liability to any of Aon Delaware, Aon UK or Aon Mergeco.

        Further, we have obtained a ruling from HMRC in respect of the stamp duty and SDRT consequences of the merger and as a result believe that we have satisfied all stamp duty and SDRT payment and filing obligations in connection with the issuance of the Class A Ordinary Shares issuable in connection with the merger.

        We have also obtained a ruling from HMRC that, following the merger, the "temporary period exemption" from the U.K.'s controlled foreign company rules will apply such that, subject to certain conditions and limitations based on our facts and circumstances, Aon UK will not be subject to tax on the profits of any controlled company which is resident in a foreign jurisdiction under the CFC rules until 24 months after the end of the accounting period in which the merger occurs, subject to any changes of legislation. The U.K. Government is expected to bring major reforms to the CFC rules before Parliament in March 2012. While HMRC cannot provide any assurance in respect of the application of legislation that has not been enacted, we are of the view based on the Government's proposals and published draft legislation that the new CFC rules should not have a material adverse effect on Aon UK's tax treatment in the U.K. if enacted in their current form. However, to the extent that the draft legislation is enacted in a form different to that currently proposed, this may result in

26

---

Table of Contents

additional corporation tax liabilities becoming payable following implementation of the revised legislation. See "Material Tax Considerations Relating to the Merger—U.K. Tax Considerations—Material U.K. Tax Considerations Relating to and After the Merger—Controlled Foreign Companies."

        Further, if HMRC disagrees with our view of any issues in respect of which no ruling has been obtained, it may take the position that material U.K. corporation tax or SDRT liabilities or amounts on account thereof are payable by any one or more of these companies as a result of the reorganization, in which case we expect that we would contest such assessment. To contest such assessment, we may be required to remit cash or provide security of the amount in dispute, or such lesser amount as permitted under U.K. law and acceptable to HMRC, to prevent HMRC from seeking enforcement actions pending the dispute of such assessment. If we were unsuccessful in disputing the assessment, the implications could be materially adverse to us. To the extent that HMRC has not provided (and we have not requested) a ruling on the U.K. tax aspects of the merger, there can be no assurance that HMRC will agree with our interpretation of the U.K. tax aspects of the merger or any related matters associated therewith.

Aon UK's net income and cash flow would be reduced if Aon UK becomes subject to U.S. corporate income tax.

        Aon UK and other non-U.S. Aon UK affiliates will conduct their operations in a manner intended to ensure that Aon UK and its non-U.S. affiliates do not engage in the conduct of a U.S. trade or business. However, if Aon UK or any of its non-U.S. affiliates is or are engaged in a trade or business in the U.S., Aon UK or such non-U.S. affiliates would be required to pay U.S. corporate income tax on income that is subject to the taxing jurisdiction of the U.S. If this occurs, our results of operations may be adversely affected. Additionally, after the merger, Aon Delaware and its then existing U.S. subsidiaries will continue to be subject to U.S. corporate income tax on their worldwide income, and Aon Delaware's then existing foreign subsidiaries will continue to be subject to U.S. corporate income tax on their U.S. operations.

The merger may not allow us to maintain a competitive global tax rate.

        We believe that the merger should significantly improve our ability to maintain a competitive global tax rate because the U.K. has implemented a dividend exemption system that generally does not subject non-U.K. earnings to U.K. tax when such earnings are repatriated to the U.K. in the form of dividends from non-U.K. subsidiaries. This should allow the Company to optimize its capital allocation and deploy efficient fiscal structures. However, we cannot provide any assurances as to what our global tax rate will be after the merger because of, among other things, uncertainty regarding the nature and extent of our business activities in any particular jurisdiction in the future and the tax laws of such jurisdictions, as well as changes in U.S. and other tax laws, treaties and regulations. Our actual global tax rate may vary from our expectation and that variance may be material. Additionally, the tax laws of the U.K. and other jurisdictions could change in the future, and such changes could cause a material change in our global tax rate.

        We also could be subject to future audits conducted by foreign and domestic tax authorities, and the resolution of such audits could impact our tax rate in future periods, as would any reclassification or other matter (such as changes in applicable accounting rules) that increases the amounts we have provided for income taxes in our consolidated financial statements. There can be no assurance that we would be successful in attempting to mitigate the adverse impacts resulting from any changes in law, audits and other matters. Our inability to mitigate the negative consequences of any changes in the law, audits and other matters could cause our global tax rate to increase and our results of operations to suffer.

27

---

Table of Contents

The IRS may disagree with our conclusions on tax treatment of the merger.

        We expect that the merger will not result in any material U.S. federal income tax liability to Aon Delaware or Aon UK. However, the IRS may disagree with our assessments of the effects or interpretation of the tax laws, treaties or regulations or their enforcement with respect to the merger. Nevertheless, even if our conclusions on the U.S. tax treatment of the merger to Aon Delaware and Aon UK do not ultimately prevail, we do not believe that a contrary treatment of the merger by the IRS would result in a material increase in U.S. taxes compared to our current, pre-merger U.S. tax position. In this event, however, we may not realize the expected tax benefits of the merger and our results of operations may be adversely affected in comparison to what they would have been if our conclusions had ultimately prevailed.

We may experience difficulty attracting and retaining colleagues as a result of the proposed merger.

        If the transactions contemplated by this proxy statement/prospectus are completed, we will relocate our principal executive offices to London, England. Following the completion of those transactions, we will need to retain those personnel we have relocated to London in that area, and, in the future, will need to attract and retain new personnel to the London area. Although we believe that London's status as a major international city will facilitate the retention and attraction of personnel, there can be no assurance that we will be successful in this regard.

Negative publicity resulting from the proposed merger could adversely affect our business and our stock price.

        Foreign reincorporations that have been undertaken by other companies have generated significant press coverage, much of which has been negative. In such situations, press coverage has been particularly negative where a company undertaking a proposed reincorporation has a historical connection to a particular U.S. locality or geographic region. Although we have grown around the world to be a global leader in many of our businesses and have significant history in the U.K., our company was founded in, and has historically been connected to, the Chicago, Illinois area.

        Negative publicity generated by the proposed merger could cause our colleagues, particularly those in the United States, generally, and the Chicago area, in particular, to perceive uncertainty regarding future opportunities available to them. In addition, negative publicity could cause some of our clients to be more reluctant to do business with us. Either of these events could have a significant adverse impact on our business. Negative publicity could also cause some of our stockholders to sell our stock or decrease the demand for new investors to purchase our stock, which could have an adverse impact on our stock price.

Our Board may choose to defer or abandon the merger at any time.

        Completion of the merger may be deferred or abandoned by action of our Board at any time, including after stockholder approval. While we currently expect the merger to take place promptly after the proposal to adopt the merger agreement is approved at the special meeting, the Board may defer completion before or after the special meeting or may abandon the merger at any time, including after stockholder approval, because of, among other reasons, changes in existing or proposed tax legislation, failure of McDermott Will & Emery LLP to deliver tax opinions in form and substance acceptable to us, our determination that HMRC does not agree with our view of the U.K. corporation tax or SDRT implications of certain aspects of the reorganization, our determination that the Class A Ordinary Shares will not be eligible for inclusion in the facilities of DTC, our determination that the IRS does not agree with our views on certain tax matters, our determination that the reorganization would involve tax or other risks that outweigh their benefits, our determination that the level of expected benefits associated with the reorganization would otherwise be reduced, a dispute with the taxation authorities over the reorganization (or certain aspects thereof), changes in U.K. or U.S. tax laws, rates,

28

---

Table of Contents

treaties or regulations that would adversely affect our ability to achieve the expected benefits of the reorganization, an unexpected increase in the cost to complete the reorganization or any other determination by our Board that the reorganization would not be in the best interests of Aon Delaware or its stockholders or that the reorganization would have material adverse consequences to Aon Delaware or its stockholders.

Your rights as a stockholder of Aon Delaware will change as a result of the merger.

        The consummation of the merger will change the governing law that applies to our stockholders from Delaware law (which applies to shares of common stock of Aon Delaware) to English law (which applies to the Class A Ordinary Shares). Some of the principal attributes of the common stock of Aon Delaware and the Class A Ordinary Shares will be similar. There are, however, several significant differences between the rights of stockholders under Delaware law and shareholders under English law, and there are differences between our current certificate of incorporation and by-laws and the proposed articles of association that will apply to our stockholders after the consummation of the merger. For a detailed discussion of these differences, see "Comparison of Rights of Stockholders/Shareholders."

As a result of increased shareholder approval requirements, we may have less flexibility as an English public limited company than as a Delaware corporation with respect to certain aspects of capital management.

        Under Delaware law, our directors may issue, without further shareholder approval, any shares authorized in our certificate of incorporation that are not already issued or reserved. Delaware law also provides substantial flexibility in establishing the terms of preferred shares. However, English law provides that a board of directors may only allot shares with the prior authorization of shareholders, such authorization being up to the aggregate nominal amount of shares and for a maximum period of five years, each as specified in the articles of association or relevant shareholder resolution. This authorization would need to be renewed by our shareholders upon its expiration (i.e., at least every five years). The articles of association that will apply to Aon UK after the effective time will authorize the allotment of additional shares, and renewal of such authorization for additional five year terms may be sought more frequently. See "Description of Class A Ordinary Shares of Aon UK—Preemptive Rights and New Issues of Shares."

        English law also generally provides shareholders with preemptive rights when new shares are issued for cash; however, it is possible for the articles of association, or shareholders in general meeting, to exclude preemptive rights. Such an exclusion of preemptive rights may be for a maximum period of up to five years from the date of adoption of the articles of association, if the exclusion is contained in the articles of association, or from the date of the shareholder resolution, if the exclusion is by shareholder resolution; in either case, this exclusion would need to be renewed upon its expiration (i.e., at least every five years). The articles of association that will apply to Aon UK after the effective time will exclude preemptive rights prior to the effective time of the merger, and renewal of such exclusion for additional five year terms may be sought more frequently. See "Description of Class A Ordinary Shares of Aon UK—Preemptive Rights and New Issues of Shares."

        English law also generally prohibits a company from repurchasing its own shares by way of "off market purchases" without the prior approval of 75 percent of shareholders by special resolution. Such approval lasts for a maximum period of up to five years. English law prohibits Aon UK from conducting "on market purchases" as its shares will not be traded on a recognized investment exchange in the U.K. We anticipate that a special resolution will be adopted to permit "off market purchases" prior to the effective time of the merger. See "Description of Class A Ordinary shares of Aon UK—Alteration of Share Capital/Repurchase of Shares." This special resolution will need to be renewed upon expiration (i.e., at least every five years) but may be sought more frequently for additional five year terms.

29

---

Table of Contents

English law will require that we meet certain additional financial requirements before we declare dividends and repurchase shares following the merger.

        Under English law, Aon UK will only be able to declare dividends, make distributions or repurchase shares out of "distributable reserves." "Distributable reserves" are a company's accumulated, realized profits, so far as not previously utilized by distribution or capitalization, less its accumulated, realized losses, so far as not previously written off in a reduction or reorganization of capital duly made. Immediately after the merger, Aon UK will not have "distributable reserves." The current shareholder of Aon UK (which is Aon Delaware) has passed a resolution to reduce the capital of Aon UK to allow the creation of distributable reserves following the merger. If the proposal to reduce Aon UK's capital is approved by Aon Delaware's stockholders and the merger is completed, we will seek to obtain the approval of the English Companies Court through a customary process, which is required for the creation of distributable reserves to be effective, as soon as practicable following the transaction. The approval of the English Companies Court is expected to be received within four weeks after the completion of the merger. If that approval is received, it is expected that Aon UK will have "distributable reserves" in an amount sufficient to continue paying quarterly dividends and to repurchase shares on the anticipated schedule for the foreseeable future.

        The approval by Aon Delaware's stockholders of the reduction of capital of Aon UK is not a condition to the completion of the merger. If stockholders do not approve the proposal to approve an implementation of a reduction of capital of Aon UK through a customary court-approved process, Aon UK would undertake other efforts to allow it to declare dividends, make distributions or repurchase shares following the completion of the merger. These efforts may include certain customary intra-group reorganizations, which are established and often-used alternatives for the creation of distributable reserves in a U.K. public limited company, but which we believe to be less advantageous than the proposed court-approved reduction in capital.

The enforcement of civil liabilities against Aon UK may be more difficult.

        Because Aon UK will be a public limited company incorporated under English law, after the effective time of the merger, investors could experience more difficulty enforcing judgments obtained against Aon UK in U.S. courts than would currently be the case for U.S. judgments obtained against Aon Delaware. In addition, it may be more difficult (or impossible) to bring some types of claims against Aon UK in courts in England than it would be to bring similar claims against a U.S. company in a U.S. court.

Following the effective time of the merger, it is possible that we will be removed from the S&P 500 stock index and other indices, which could have an adverse impact on our share price.

        Our shares currently are a component of the Standard & Poor's 500 Index and other indices. Based on current S&P guidelines, we believe that our Class A Ordinary Shares meet S&P criteria for inclusion in the S&P 500 Index upon completion of the merger, which criteria include requirements that its constituent companies file Annual Reports on Form 10-K, are not foreign private issuers, derive a plurality of revenues or have a plurality of fixed assets in the U.S., are primarily listed on the NYSE or NASDAQ and have a corporate governance consistent with U.S. practice. Nonetheless, S&P retains discretion to determine whether our shares should continue as a component of the S&P 500 upon the completion of the merger. We anticipate that S&P will determine whether the Class A Ordinary Shares qualify for inclusion on or around the effectiveness of the merger. However, following the effective time of the merger, it is possible that, if we were to fail to meet any of S&P's guidelines for inclusion in the S&P 500, our Class A Ordinary Shares would be excluded from the index. If our Class A Ordinary Shares are not included as a component of the S&P 500 or other indices or no longer meet the qualifications of such funds, institutional investors that are required to track the performance of the S&P 500 or such other indices or the funds that impose those qualifications would potentially be

30

---

Table of Contents

required to sell their Class A Ordinary Shares, which could adversely affect the price of the Class A Ordinary Shares.

The market for Class A Ordinary Shares may differ from the current market for Aon Delaware shares.

        Although it is anticipated that the Class A Ordinary Shares will be authorized for listing on the NYSE under the symbol "AON," which is the same symbol under which shares of Aon Delaware are currently listed, the market prices, trading volume and volatility of the Class A Ordinary Shares could be different from those of the Aon Delaware shares.

Transfers of the Class A Ordinary Shares may be subject to stamp duty or SDRT in the U.K., which would increase the cost of dealing in the Class A Ordinary Shares as compared to the Aon Delaware shares.

        Stamp duty and/or SDRT are imposed in the U.K. on certain transfers of chargeable securities (which include shares in companies incorporated in the U.K.) at a rate of 0.5 percent of the consideration paid for the transfer. Certain issues or transfers of shares to depositaries or into clearance systems are charged at a higher rate of 1.5 percent.

        You are strongly encouraged to hold your Class A Ordinary Shares in book entry form through the facilities of DTC. Transfers of shares held in book entry form through DTC will not attract a charge to stamp duty or SDRT in the U.K. A transfer of title in the shares from within the DTC system out of DTC and any subsequent transfers that occur entirely outside the DTC system, including repurchase by Aon UK, will attract a charge to stamp duty at a rate of 0.5 percent of any consideration, which is payable by the transferee of the shares. Any such duty must be paid (and the relevant transfer document stamped by HMRC) before the transfer can be registered in the books of Aon UK. But if those shares are redeposited into DTC, the redeposit will attract stamp duty or SDRT.

        Following the merger, Aon UK expects to put in place arrangements to require that shares held in certificated form cannot be transferred into the DTC system until the transferor of the shares has first delivered the shares to a depository specified by Aon UK so that SDRT may be collected in connection with the initial delivery to the depository. Any such shares will be evidenced by a receipt issued by the depository. Before the transfer can be registered in the books of Aon UK, the transferor will also be required to put in the depository funds to settle the resultant liability to SDRT, which will be charged at a rate of 1.5 percent of the value of the shares.

        Following the merger, SDRT will be imposed upon the issue of shares in settlement of equity-based awards under our stock or share incentive plans. This could increase our costs to continue such plans as currently designed and utilized. Following the merger, SDRT may also be imposed on other future issuances of our stock, increasing the costs of engaging in those transactions. Aon UK expects to put in place arrangements to pay any stamp duty or SDRT before transferring the newly issued shares to DTC, thereby enabling future issuances of Class A Ordinary Shares to be transferred into the DTC system without incurring further SDRT or stamp duty.

        The enforceability of the 1.5 percent charge is in doubt and is the subject of ongoing litigation following the decision of the European Court of Justice in HSBC Holdings plc, Vidacos Nominees Ltd v HMRC Case C-569/07. It is possible that the U.K. government may change the law in relation to stamp duty and SDRT in response to this litigation, and that this will have a material effect on the cost of dealing in the Aon UK shares.

If the Class A Ordinary Shares are not eligible for deposit and clearing within the facilities of DTC, then transactions in our securities may be disrupted.

        The facilities of DTC are a widely-used mechanism that allow for rapid electronic transfers of securities between the participants in the DTC system, which include many large banks and brokerage

31

---

Table of Contents

firms. We believe that approximately 98% of the outstanding shares of common stock of Aon Delaware are currently held within the DTC system. We expect that, upon the consummation of the merger, the Class A Ordinary Shares of Aon UK will be eligible for deposit and clearing within the DTC system. However, while we are aware of a number of U.K. companies whose ordinary shares trade in the United States in the form of American Depository Receipts, we are not aware of any other U.K. company whose ordinary shares are directly traded on a U.S. securities exchange and are cleared through the DTC system. We believe that this is, in part, a result of concern on the part of DTC regarding its potential liability for stamp duty and/or SDRT in connection with transactions in ordinary shares of U.K. companies. In connection with the closing of the merger, we expect to enter into arrangements with DTC whereby we will agree to indemnify DTC for any stamp duty and/or SDRT that may be assessed upon it as a result of its service as a depository and clearing agency for our Class A Ordinary Shares. In addition, we have obtained a ruling from HMRC in respect of the stamp duty and SDRT consequences of the reorganization, and SDRT has been paid in accordance with the terms of this ruling in respect of the deposit of Class A Ordinary Shares with EES. We expect these actions, among others, will result in DTC agreeing to the accept the Class A Ordinary Shares for deposit and clearing within its facilities upon consummation of the merger.

        DTC is not obligated to accept the Class A Ordinary Shares for deposit and clearing within its facilities at the closing and, even if DTC does initially accept the Class A Ordinary Shares, it will generally have discretion to cease to act as a depository and clearing agency for the Class A Ordinary Shares. If DTC determined prior to the consummation of the merger that the Class A Ordinary Shares are not eligible for clearance within the DTC system, then we would not expect to complete the transactions contemplated by this proxy statement/prospectus in their current form. However, if DTC determined at any time after the consummation of the merger that the Class A Ordinary Shares were not eligible for continued deposit and clearance within its facilities, then we believe the Class A Ordinary Shares would not be eligible for continued listing on a U.S. securities exchange or inclusion in the S&P 500 and trading in the Class A Ordinary Shares would be disrupted. While we would pursue alternative arrangements to preserve our listing and maintain trading, any such disruption could have a material adverse effect on the trading price of the Class A Ordinary Shares.

Our executive officers and directors have interests in the merger that are different from, or in addition to, the interests of our stockholders.

        Aon Delaware's executive officers and directors have financial interests in the merger that are different from, or in addition to, their interests as Aon Delaware stockholders. In particular, Aon Delaware's executive officers who will relocate to the UK in connection with the transactions contemplated by this proxy statement/prospectus will receive relocation benefits pursuant to international assignment letter agreements that we have signed with them. For more information, see "Proposal 1—Approval of the Adoption of the Merger Agreement—Interests of Directors and Executive Officers in the Merger."

        In addition, in connection with the merger, Aon UK intends to enter into customary indemnification agreements with the directors and officers of Aon UK.

If we are not successful in amending or waiving certain provisions of our bank credit facilities, the merger may result in a breach of certain provisions of these facilities.

        In connection with the reorganization, we expect to seek the amendment or waiver of certain provisions of, or to renegotiate the terms of, the US Term Facility, the US Revolving Facility and the Euro Revolving Facility. For more information regarding these facilities, see "Proposal 1—Approval of the Adoption of the Merger Agreement—Additional Agreements and Actions." If we are unsuccessful in modifying these credit facilities by the effective time of the merger, we may be in breach of certain provisions of these credit facilities. Although we believe that we will be successful in obtaining any

32

---

Table of Contents

necessary modifications of our credit facilities, we can offer no assurance as to what additional or different terms or conditions, if any, our lenders may request in connection with those modifications. Any such changes in terms or conditions could result in an increase in our borrowing costs or a decrease in our financial or operating flexibility.

We expect to incur transaction costs in connection with the completion of the reorganization, some of which will be incurred whether or not the reorganization is completed.

        We have begun to incur and expect to incur in 2011 and 2012 a total of approximately $20 million in transaction costs in connection with the reorganization. The substantial majority of these costs will be incurred regardless of whether the reorganization is completed and prior to your vote on the proposal.

        Following the merger and in connection with the reorganization, we plan to examine various potential transactions for the further repositioning of the organizational structure of Aon Delaware and certain of its subsidiaries. We refer to these activities and transactions in this proxy statement/prospectus as "subsequent actions." Aon Delaware could recognize gain, and be subject to U.S. federal income tax on any such gain, as a result of one or more of these transactions. The amount of income taxes incurred in connection with any transactions will depend on a number of factors. Based on information currently available, we do not expect any transactions to have a significant impact on our reported income tax expense.

        In connection with the completion of the reorganization, we will reevaluate the ability to realize our deferred tax assets related to U.S. operations under the new Aon UK corporate structure and we may recognize a non-cash, deferred tax expense upon the conclusion of this evaluation. Based on information currently available, we do not expect the additional deferred tax expense, if any, to be significant.

The reorganization will result in additional ongoing costs to us.

        The completion of the reorganization will result in an increase in some of our ongoing expenses and require us to incur some new expenses. Some costs, including those related to employees in our U.K. offices and holding board meetings in the U.K., are expected to be higher than would be the case if our principal executive offices were not relocated to the U.K.. We also expect to incur new expenses, including professional fees and SDRT in connection with settlement of equity-based awards under our stock or share incentive plans, to comply with U.K. corporate and tax laws.

33

---

Table of Contents

CAUTIONARY STATEMENT CONCERNING FORWARD-LOOKING STATEMENTS

        This proxy statement/prospectus contains certain statements related to future results, or states our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. Forward-looking statements relate to expectations or forecasts of future events. They use words such as "anticipate," "believe," "estimate," "expect," "forecast," "project," "intend," "plan," "potential," and other similar terms, and future or conditional tense verbs like "could," "may," "might," "should," "will" and "would." You can also identify forward-looking statements by the fact that they do not relate strictly to historical or current facts. For example, we may use forward-looking statements when addressing topics such as the reorganization and our plans, objectives, expectations and intentions with respect thereto and with respect to future operations, including the benefits and tax savings or impact described in this proxy statement/prospectus that we expect to achieve as a result of the reorganization. Forward-looking statements may also include statements regarding market and industry conditions, including competitive and pricing trends; changes in our business strategies and methods of generating revenue; the development and performance of our services and products; changes in the composition or level of our revenues; our cost structure and the outcome of cost-saving or restructuring initiatives; the outcome of contingencies; dividend policy; the expected impact of acquisitions and dispositions; pension obligations; cash flow and liquidity; future actions by regulators; and the impact of changes in accounting rules. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include:

•

the merger may not be approved by our stockholders;



•

our Board may choose to defer or abandon the merger at any time;



•

changes in foreign or domestic laws, including tax laws, or other regulatory actions, orders or rulings by foreign or domestic governmental entities, that could effectively preclude us from completing the merger or reduce or eliminate the benefits we expect to achieve from the reorganization;



•

negative publicity resulting from the proposed merger having an adverse effect on our business;



•

an SEC stop order or other action or any other decree, order or injunction preventing the registration statement of which this proxy statement/prospectus is a part from becoming or remaining effective or preventing us from holding the special meeting or completing the merger;



•

an inability to satisfy all of the conditions to closing set forth in the merger agreement;



•

an inability to realize expected benefits from the merger or the occurrence of difficulties in connection with the merger, or a delay in the realization of such benefits, including as a result of possible turnover among our U.S. colleagues or loss of U.S. clients;



•

costs related to the merger, which could be greater than expected;



•

our possible removal from the S&P 500 stock index;



•

the possibility that our Class A Ordinary Shares will not be or will cease to be eligible for acceptance by DTC or that their eligibility is subject to additional expenses;



•

general economic conditions in different countries in which we do business around the world;



•

changes in the competitive environment;



•

changes in global equity and fixed income markets that could influence the return on invested assets;

34

---

Table of Contents

•

changes in the funding status of our various defined benefit pension plans and the impact of any increased pension funding resulting from those changes;



•

rating agency actions that could affect our ability to borrow funds;



•

fluctuations in exchange and interest rates that could impact revenue and expense;



•

the impact of class actions and individual lawsuits including client class actions, securities class actions, derivative actions and ERISA class actions;



•

the impact of any investigations brought by regulatory authorities in the U.S., U.K. and other countries;



•

the cost of resolution of other contingent liabilities and loss contingencies, including potential liabilities arising from errors and omission claims against us;



•

failure to retain and attract qualified personnel, including potential difficulties in executive succession-planning related to our need to attract talent to London if the merger is completed;



•

the impact of, and potential challenges in complying with, legislation and regulation in the jurisdictions in which we operate, particularly given the global scope of our business and the possibility of conflicting regulatory requirements across jurisdictions in which we do business;



•

the extent to which we retain existing clients and attract new businesses and our ability to incentivize and retain key employees;



•

the extent to which we manage certain risks created in connection with the various services, including fiduciary and advisory services, among others, that we currently provide, or will provide in the future, to clients;



•

our ability to maintain the security and privacy of confidential information belonging to our clients, and in the case of our HR Solutions clients, our clients' current and former employees;



•

our ability to innovate and keep pace with rapid and continuing changes in technology, industry standards and client preferences;



•

disruption from the merger with Hewitt making it more difficult to maintain business and operational relationships;



•

the possibility that the expected efficiencies and cost savings from the merger with Hewitt will not be realized, or will not be realized within the expected time period;



•

the risk that the Hewitt businesses will not be integrated successfully;



•

our ability to implement restructuring initiatives and other initiatives intended to yield cost savings, and the ability to achieve those cost savings;



•

our implementation of changes to the methods in which we internally process and monitor transactions;



•

changes in commercial property and casualty markets and commercial premium rates that could impact revenues;



•

the outcome of any inquiries from regulators relating to compliance with the U.S. Foreign Corrupt Practices Act and non-U.S. anti-corruption laws, such as the U.K. Bribery Act 2010; and



•

changes in costs or assumptions associated with our HR Solutions' outsourcing and consulting arrangement that affect the profitability of these arrangements.

        Any or all of our forward-looking statements may turn out to be inaccurate, and there are no guarantees about our performance. The factors identified above are not exhaustive. Aon Delaware and

35

---

Table of Contents

its subsidiaries operate in a dynamic business environment in which new risks may emerge frequently. Accordingly, readers should not place undue reliance on forward-looking statements, which speak only as of the dates on which they are made. We are under no obligation (and expressly disclaim any obligation) to update or alter any forward-looking statement that we may make from time to time, whether as a result of new information, future events or otherwise. Further information about factors that could materially affect Aon, including our results of operations and financial condition, is contained in the "Risk Factors" section in Part I, Item 1A of our Annual Report on Form 10-K for the year ended December 31, 2010.

36

---

Table of Contents

PROPOSAL 1—APPROVAL OF THE ADOPTION OF THE MERGER AGREEMENT

        The following includes a brief summary of the material provisions of the merger agreement, a copy of which is attached as Annex A and incorporated by reference into this proxy statement/prospectus. We encourage you to read the merger agreement in its entirety for a more complete description of the merger. In the event of any discrepancy between the terms of the merger agreement and the following summary, the merger agreement will control.

The Merger Agreement

Introduction

        The merger agreement you are being asked to adopt at the special meeting provides for a merger that would result in your shares of Aon Delaware common stock being converted into the right to receive Class A Ordinary Shares of Aon UK which will be, at the effective time of the merger, a public limited company incorporated under English law. Aon UK, together with its subsidiaries, will thereafter own and continue to conduct our business in substantially the same manner as is currently being conducted by Aon Delaware and its subsidiaries. Immediately following the merger, you will have the right to receive Class A Ordinary Shares of Aon UK, which will be managed by the same board of directors and executive officers that manage Aon Delaware immediately prior to the merger. Additionally, the consolidated assets and business of Aon UK immediately after the merger will be the same as those of Aon Delaware immediately prior to the merger.

        Under the merger agreement, Aon Mergeco, a wholly-owned subsidiary of Aon Intermediate (which itself is currently a wholly-owned subsidiary of Aon Delaware) will merge with and into Aon Delaware, with Aon Delaware surviving the merger as a wholly-owned subsidiary of Aon Intermediate. If the merger agreement is adopted by the stockholders, we anticipate that the merger will become effective shortly after such approval, subject to the making or receipt of applicable regulatory filings, registrations, consents and approvals. We currently anticipate that the merger will become effective in the second quarter of 2012.

The Parties to the Merger

        Aon Delaware provides risk management services, insurance and reinsurance brokerage, and human resource consulting and outsourcing, delivering distinctive client value via innovative and effective risk management and workforce productivity solutions. Aon Delaware delivers its technical expertise locally through colleagues worldwide.

        We serve clients through the following businesses:

•

Risk Solutions acts as an advisor and insurance and reinsurance broker, helping clients manage their risks via consultation, as well as negotiation and placement of insurance risk with insurance carriers through our global distribution network.



•

HR Solutions partners with organizations to solve their most complex benefits, talent and related financial challenges, and improve business performance by designing, implementing, communicating and administering a wide range of human capital, retirement, investment management, health care, compensation and talent management strategies.

        Our clients include corporations and businesses, insurance companies, professional organizations, independent agents and brokers, governments, and other entities. We also serve individuals through personal lines, affinity groups, and certain specialty operations.

        Aon Mergeco is a newly formed Delaware corporation and currently is a wholly-owned subsidiary of Aon Intermediate, which is a newly formed Delaware limited liability company and currently is a wholly-owned subsidiary of Aon Delaware. Neither Aon Intermediate nor Aon Mergeco has a

37

---

Table of Contents

significant amount of assets or liabilities and neither has engaged in any business since its formation other than activities associated with its anticipated participation in the merger.

        The principal executive offices of Aon Delaware, Aon Intermediate and Aon Mergeco in the U.S. are currently located at 200 East Randolph Street, Chicago, Illinois 60601, U.S.A., and the telephone number of each company is (312) 381-1000.

        The principal executive offices of Aon UK are currently located at 8 Devonshire Square, London EC2M 4PL, England, and its telephone number is +44 20 7623 5500.

Background and Reasons for the Merger

Background

Current U.K. Operations

        Aon has had a long-standing history and presence in the U.K. Aon was created in 1982 when the Ryan Insurance Group merged with Combined International Corporation. In March of 1987, the shareholders voted to change Combined International Corporation's name to Aon. Aon expanded rapidly during the 1980s and 1990s through several strategic acquisitions that enabled the company to reposition itself as a premier global insurance and reinsurance broker with close ties to the Lloyd's market. Aon acquired numerous brokers during this period with a strong Lloyd's heritage, one of the oldest having been a Lloyd's broker since 1823. Among the UK brokers acquired by Aon during this period were: Agricultural Risk Management, Asset Security Managers, Bain Hogg, Bishopsgate, Jenner Fenton Slade, Live Wire Group, Minet, Nicholson, Chamberlain & Colls, NYA Brand, Open & Direct Group Limited (Footmann James), Reed Stenhouse, Sinser Holdings AB, Supercover, Surety & Guarantee Consultants Ltd, Water for Fish, and Z Yen Limited. In addition, Aon acquired several global brokers that had a substantial UK presence, including Alexander & Alexander Services Inc., Frank B. Hall, Gallagher Re and Benfield. Over its recent history, Aon has also acquired several consulting firms with substantial UK operations, including, Surety & Guarantee Consultants Ltd., Water for Fish, Z Yen Limited, Live Wire Group, Agricultural Risk Management and NYA Brand.

        London continues to be a key geographic center of our global business. Aon has a substantial retail insurance brokerage business that places insurance on behalf of its UK-based clients. In addition, our Risk Solutions business currently places coverage in London for a large number of risks on behalf of non-UK-based clients requiring the expertise and breadth of the insurance offerings available in the mature London market. While certain basic insurance policies can be placed locally in many jurisdictions around the world, we will typically place complex or large policies in London. For example, a client in Houston may be able to easily secure basic property insurance on a small office building from a local carrier. However, that same client would be unable to insure its portfolio of oil platforms in the Gulf of Mexico, as the expertise and financial backing to cover that risk exists almost entirely in London. In its role as that client's broker, the local Aon affiliate will work with Aon brokers stationed in the Lloyd's of London market to arrange for the policy to be placed. In addition, the Lloyd's market operates through syndicates of global carriers, which share the relevant risks related to the placement. This allows Aon clients to access a diverse carrier listing which bear global risks, and are less impacted by local factors.

        Lloyd's of London has a long tradition of being the most exclusive and preeminent insurance and reinsurance market in the world, having developed over the last three hundred years through the interactions between brokers and underwriters. Lloyd's remains a relationship-driven operation that requires face-to-face interaction between brokers and underwriters. Business is conducted in the underwriting room at the Lloyd's Building, where brokers have immediate access to underwriters to negotiate and agree on pricing for the policies placed on behalf of clients. As most of the risks placed through Lloyd's are often large, complex, and highly individualized and large-scale, in-person

38

---

Table of Contents

negotiations are critical to successfully placing insurance and reinsurance policies. Maintaining a close connection with the insurance underwriters that operate at the Lloyd's market continues to be an important driver of our business.

        In addition, we have a number of specialty groups that make a substantial proportion of their placements in London. These include the Energy, Power, Mining, Marine, Aviation, Construction, Crisis Management, and Professional Services Groups. Each of these businesses deals in substantially complicated, and typically large, insurance policies for which specialty knowledge is required and an experienced market like Lloyd's is essential.

        The London market is also a center of our reinsurance brokerage operations. The reinsurance brokerage industry developed in London in the late 1800s, with a principal contributor being Cuthbert Heath, the "Founder of Modern Lloyd's." Reinsurance brokerage facilitated expansion of Lloyd's insurers into foreign markets at the turn of the twentieth century by allowing them to alleviate their risks in unfamiliar social and political geographies. London reinsurance brokers continue to be the principal developers of emerging markets, particularly in Eastern Europe and Asia. Aon Re already had substantial London business operations when it acquired UK-headquartered Benfield—at the time the world's third-largest reinsurance broker—to create the preeminent reinsurance brokerage firm in the world. Benfield's corporate ancestry dates back to the founding of W.T. Greig Ltd. in 1921, the company credited as the principal innovator of the London reinsurance brokerage industry with its development of catastrophic analytics and overseas expansion. Aon Re is therefore well positioned in the important London market where a disproportionate amount of Aon Re's business continues to be placed for non-UK and UK clients. London is a critical reinsurance center both because of the large number of clients based in London as well as the presence of specialist capability which means a disproportionate amount of business is placed through London.

        The United Kingdom is also a central location for our HR Solutions business. Throughout the 2000s, the business associated with HR Solutions, under the Hewitt name, acquired a number of companies with substantial operations in London. In 2000, Aon acquired Bacon & Woodrow, the third-largest employee benefits consulting firm in the U.K. By 2003, over 10% of Hewitt's employees were located in the U.K. In 2004, Hewitt made its largest acquisition, the purchase of Exult Inc., a publically-traded company specializing in multi-process HR Outsourcing (now called HR BPO). Exult's international headquarters were located in the U.K., and its core HR BPO service lines, such as payroll and HR information systems, were natural adjacencies to Hewitt's core benefits business. This acquisition accelerated Hewitt's existing market presence both within the HR BPO space as well as internationally through the London presence. In 2008, Hewitt acquired New Bridge Street Consultants, a U.K. based consultancy that assists companies in designing and implementing remuneration policies that help them meet their strategic goals. As a result of these acquisitions, London has been established as a key international market for each of HR Solutions' business lines of consulting, benefits administration and HR BPO.

        In 2010, Aon Hewitt earned $4.5 billion in revenue on a pro forma basis. Of this amount, roughly $2.2 billion is from Aon Hewitt's Benefits Administration and BPO businesses and $2.3 billion is from its consulting business. Of the $2.3 billion in consulting revenues, $1.2 billion is earned internationally and over $400 million was earned in the U.K.

        Overall, our U.K. business accounted for approximately 15.5% of our 2010 revenue.

Expansion of our Global Presence

        Over the last 20 years, Aon has transformed its profile from a US-centric firm to a more global professional services firm. Our revenue mix from international operations has increased significantly during this period—from 13% of global revenues in 1990 to 52% of global pro forma revenues in 2010.

39

---

Table of Contents

        Aon drives its capital decision making process around return on invested capital, firmly believing this methodology has a higher correlation than any other metric with creation of shareholder value. In this regard:

•

Beginning in 2006, we began to simplify our global organization and execute on a strategy to exit lower margin and more capital intensive insurance underwriting businesses.



•

From 2006 to 2009, Aon divested significant non-core assets including: Aon Warranty Group, Construction Program Group, Sterling Life Insurance, and Combined Insurance Companies of America, each of which had substantial US operations.



•

The successful exit of the low margin, capital intensive insurance underwriting businesses and non-core assets strategically eliminated 23% of the company's total revenue in 2006.

        Then, with the acquisitions of Benfield Group in 2008 and Hewitt Associates, Inc. in 2010, Aon successfully transformed its portfolio to higher-margin, capital-light professional services businesses with 75% of revenues in Risk Solutions and 25% of total revenues in HR Solutions, resulting in high recurring revenue streams and strong free cash flow generation characteristics. For the nine months ended September 30, 2011, 60% of our revenues were earned by Risk Solutions and 40% by HR Solutions. Aon expects to experience a growing international presence in both segments.

Reasons for the Merger

        The merger is part of a reorganization of Aon's corporate structure, which includes a change of our place of incorporation from Delaware to the U.K., where we already have substantial operations, and the relocation of several key executive officers with global responsibilities to London.

        We believe the reorganization will strengthen our long-term strategy by:

•

Enhancing our focus on growth by positioning our resources more proximately to the geographic areas poised to experience the highest level of expansion and which areas today derive a substantial portion of their risk coverage through the U.K. market;



•

Enabling our Risk Solutions business to deliver superior value to our clients and differentiate us from our competition via key retail initiatives that are or will be substantially driven from the U.K., such as GRIP, Programs & Facilities and Broking Excellence and key reinsurance initiatives that are or will be substantially driven from the U.K., such as facultative programs and further expanding our Aon Re Analytics capabilities;



•

Allowing us to expand the penetration of our HR Solutions segment into Europe through London, the current base for much of our European consulting operations and growing our U.K. and EMEA benefits administration and HR BPO business lines;



•

Leveraging our development, retention and acquisition of professional talent and optimizing its geographic deployment around the world in each of our primary and developing markets;



•

Optimizing our fiscal planning and capital allocation and reducing our global tax rate in a manner that provides us with increased financial flexibility to properly invest in our growth, Risk Solutions product innovation, HR Solutions expansion and talent development strategies while delivering increased shareholder value as follows:



•

Providing greater global access, as a result of the U.K.'s tax rules regarding the repatriation of earnings, to expected increases in future cash flow as Aon delivers on its long-term operating margin targets while certain required uses of cash decrease over the next several years as restructuring costs decline and contributions to our material pension plans that are closed to new entrants and, where statutorily permitted, frozen from continued benefit accruals are reduced, as those plans become more fully funded;

40

---

Table of Contents

•

Enabling us to access approximately $300 million of excess capital held internationally on our balance sheet upon consummation of this reorganization;



•

Increasing future cash flows through a significant reduction in our global tax rate over the long term due to changes in the geographic distribution of income, similar to the reduction achieved over the last five years, which would allow us to remain competitive with certain global competitors; and



•

Delivering increased shareholder value as we effectively allocate capital through share repurchases and dividends.

Growth

        We expect our revenues outside of North America to increase as a proportion of our overall revenues and we believe that this growth will come disproportionately from Latin America, Asia, Africa and Eastern Europe, and is correlated with growth in insurance premium penetration, increased need for broad based human resources solutions, higher GDP growth and high employment rates.

        The reorganization is intended to position our company to further benefit from these growth opportunities and allow our management to more effectively implement our global strategy and increase our focus on customer development in these areas by positioning more resources closer to these areas. In this regard, we also expect that the reorganization will result in operational and administrative efficiencies over the long term and enhance our ability to expand in the U.K., Europe and internationally, while maintaining our focus on our critical U.S. market as well as better serving our large U.S. multinational customers.

Risk Solutions

        Our broking strategies are expected to drive growth in our Risk Solutions business through product and broking innovations such as the Global Risk Insight Platform, Programs & Facilities, retail Broking Excellence, and reinsurance broking. In particular:

•

Our GRIP product offers world class benchmarking for all complex or large risks placed through the London market. Through continually enhancing and expanding these GRIP activities with our Lloyd's carriers, we can continue to develop new products and best in class broking solutions for our clients.



•

Our broking activities in London are key to our Programs & Facilities strategy as they allow us to continually provide innovative best in class solutions underwritten out of London to address new and existing risks for our clients at the right price with the best coverage. These solutions often need the capacity of the London market and the capability of the people to design and develop such facilities in a way that meet the global needs of the network offices serving local clients.



•

Our retail Broking Excellence strategy is centered out of our Global Broking Centre ("GBC") in London providing solutions in the many specialist lines and wholesale markets for our global clients' large and complex risks. Expanding and investing in the capability and expertise in the GBC is critical to our ability to continue delivering these best in class program design solutions within this Broking Excellence strategy. Residing close to Lloyd's enables access to both the talent resource pool and specialist capability that is needed to achieve this.



•

Our reinsurance broking strategy depends heavily on the expertise and specialist knowledge provided by the Lloyd's market and London based capital providers of global reinsurers. It is essential we utilize this specialist capability and London talent to be able to innovate and deliver

41

---

Table of Contents

robust solutions and analytics that meet our insurance and reinsurance clients' business strategy needs and their balance sheet protection needs.

        To efficiently develop these innovations for clients, there is a need for their supporting teams to be close to Lloyd's of London and the specialist capability and talent that exists among Aon's UK brokers. London has historically been a global center for training brokers, and Benfield has a long-standing tradition for training reinsurance brokers from brokerage firms in Asia, Europe and the United States. As a result, teams responsible for developing and supporting these innovations are centered in London. We expect that locating more executive resources in London will increase and improve our access to the insurance markets there as well as appropriately leveraging our technical capabilities and talent in the London office.

HR Solutions

        Through our HR Solutions business we expect to be a leading consulting and benefits outsourcing service provider in all markets in the world. London is a key market for each of the Aon Hewitt business lines of consulting, benefits administration and HR Business Process Outsourcing. Within the consulting line, London is the base for the European business which governs emerging Eastern European, Middle Eastern and African markets where Aon Hewitt has identified future growth potential. The retirement benefits consulting industry is similarly based out of London, with substantial Aon Hewitt human capital working with key European and global clients. The investment consulting business is also headquartered in London, with research and data analytics being performed for pension plans around the world. London is also a key market for the global benefits administration and business process outsourcing business. Administration of retirement and flexible benefits, the largest line of business in the administration industry, is principally performed out of London for all of Europe, and back office functions performed in Poland are directed out of London.

Talent Development and Deployment

        We believe that a substantial investment in talent development across Aon will be central to our success over the next twenty years. A considerable effort is already underway, entitled "Aon United Talent." This initiative establishes a global approach to talent development, retention and acquisition. Being the "destination of choice" for the best talent in Risk and HR is fundamental to our delivering the best professional services to clients. This effort will also result in additional hiring and investment in offices of regional significance to Aon, including London, Chicago and Singapore.

        Given the importance of London and the U.K. to our business, we already have several key senior executives who are located there or have been trained in our London office. As a result of this reorganization, certain additional key senior executive positions are expected to relocate to London. Our London office will continue to be a global training center for our executives and brokers, as well as our Chicago and Singapore offices. We expect that having more senior executives located in a more central location will permit them to coordinate and interact with worldwide operations during their normal business hours, and improve executive oversight of these operations.

Financial Flexibility

        The reorganization will provide the additional benefit of allowing us the opportunity to optimize our fiscal planning and capital allocation in a manner that allows us to better utilize our capital, including to support and further enable our strategies to drive future growth, enhance product innovation in our Risk Solutions segment, expand the geographic reach of our HR Solutions segment, and spur talent development. It will create financial flexibility for us in four primary ways:

•

The reorganization will provide us with greater global access, as a result of the U.K.'s tax rules regarding the repatriation of earnings, to expected increases in future cash flow as Aon delivers

42

---

Table of Contents

on its long-term operating margin targets while certain required uses of cash decrease over the next several years; as restructuring costs decline and contributions to our material pension plans that are closed to new entrants and where statutorily permitted, frozen from continued benefit accruals, are reduced as those plans become more fully funded.

•

Upon its consummation, the reorganization will also allow us to access approximately $300 million of excess capital held internationally on our balance sheet.



•

We expect that due to changes in the geographic distribution of our income after the reorganization, future cash flows should increase as our global tax rate is significantly reduced.



•

The improvement in capital allocation should enable us to deliver increased shareholder value through share repurchases and dividend distributions.

        While we expect that the reorganization should improve our financial flexibility along these lines, we cannot give any assurance as to what the scope or magnitude of the improvement in our capital position will be after the reorganization. Nor can we give any assurances that tax laws in the U.K. or the U.S. will not be modified or interpreted in a manner that adversely impacts our ability to avail ourselves of these benefits.

        While the reasons set forth above were the principal rationale for our Board's approval of the reorganization, the Board also took into account that the U.K. is a major financial center known for its stability and financial sophistication and that the U.K. has a stable and well-developed legal system that we believe encourages high standards of corporate governance and provides shareholders with substantial rights. See "Description of Class A Ordinary Shares of Aon UK," and "Comparison of Rights of Stockholders/Shareholders."

        In approving the merger, our Board was also cognizant of and considered a variety of negative or potentially negative factors, including the possibility that the uncertainty created by the relocation of our legal domicile and potential negative press coverage associated with that relocation and relocation of certain key executives could create uncertainty among our colleagues in the U.S. generally, and Chicago, specifically; the possibility that such negative coverage could make some of our clients, particularly those in the U.S., reluctant to do business with us; the capital gains tax that will affect some of our stockholders; the fact that we expect to incur costs to complete the reorganization, including the costs to relocate colleagues to the U.K., and the costs to further invest in our U.K. presence; the fact that U.K. corporate law imposes additional obligations on the Company and that compliance with applicable U.K. corporate regimes will result in increased costs worldwide; the diversion of management's time and attention; and the fact that the expected benefits of the reorganization may not be realized for a variety of reasons or delayed for a significant period of time, including as a result of authorities disagreeing with our conclusions on tax treatment or changing applicable tax laws and regulations with potentially retroactive effect.

        In the course of its review of the reorganization and its attendant risks, our Board engaged in discussions with management and external advisors to evaluate and determine methods to mitigate potentially negative consequences of the reorganization. Please see "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" for further discussion of the negative and potentially negative factors discussed above and for a discussion of factors that could prevent us from achieving the benefits described above that we anticipate from completion of the reorganization.

        After completing its process of review of the expected benefits and potential risks, our Board determined, with its members Edgar J. Jannotta, Andrew J. McKenna and John W. Rogers, Jr. abstaining and each of the other members of the Board voting in favor, that the merger, the reorganization and the passage of each of the proposals contained in this proxy statement is in the best interests of Aon Delaware and its stockholders and to recommend that Aon Delaware stockholders

43

---

Table of Contents

should vote in favor of the adoption of the merger agreement and each of the other proposals contained in this proxy statement.

The Merger

        The steps that have been taken to date, and that will be taken, to complete the merger are:

•

Aon Delaware formed Aon UK as an English private limited company, with Class A Ordinary Shares and Class B Ordinary Shares.



•

Aon Delaware deposited the Class A Ordinary Shares of Aon UK with EES. Aon Delaware and EES entered into a depositary agreement governing the deposit of the Class A Ordinary Shares with EES. Aon Delaware continued to hold the Class B Ordinary Shares.



•

Aon Delaware formed Aon Intermediate as a Delaware limited liability company and a direct wholly-owned subsidiary of Aon Delaware.



•

Aon Intermediate formed Aon Mergeco as a Delaware corporation and a direct wholly-owned subsidiary of Aon Intermediate.



•

Aon Delaware and Aon Mergeco entered into the merger agreement;



•

The stockholders of Aon Mergeco adopted and approved the merger agreement;



•

Aon Delaware and Aon Intermediate entered into a contribution agreement whereby Aon Delaware agreed that, conditional upon the effectiveness of the registration statement of which this proxy statement/prospectus is a part, Aon Delaware will contribute all of its interests in the Class A Ordinary Shares to Aon Intermediate. Aon Intermediate and Aon Mergeco entered into a contribution agreement whereby Aon Intermediate agreed that, conditional upon its receiving such interests in the Class A Ordinary Shares, Aon Intermediate will contribute all of its interests in the Class A Ordinary Shares to Aon Mergeco.



•

Aon Delaware and Aon UK entered into a contribution agreement whereby Aon Delaware agreed that, conditional upon approval of the merger agreement by its stockholders, Aon Delaware will contribute all shares of Aon Intermediate to Aon UK.



•

Prior to the effective time of the merger, Aon UK will re-register as a public limited company and change its name to "Aon plc" or a similar name.



•

A special resolution of Aon UK will be passed to alter the rights of the holders of the Class A Ordinary Shares to have the same rights as holders of Class B Ordinary Shares, conditional upon the merger becoming unconditional in all respects.



•

After the stockholders of Aon Delaware approve the adoption of the merger agreement, Aon Delaware will contribute all shares of Aon Intermediate to Aon UK.



•

When the merger becomes effective,



•

Each share of Aon Delaware common stock (other than treasury shares) will be converted by operation of law into and become the right to receive one Class A Ordinary Share. This will result in each holder of Aon Delaware common stock receiving one Class A Ordinary Share for each share of Aon Delaware common stock held immediately prior to the merger;



•

Each holder of Aon Delaware common stock exchanged in the merger who would otherwise have received a fraction of a Class A Ordinary Share will receive cash in an amount determined by multiplying the fractional interest to which such holder would otherwise be entitled by the closing price for a share of Aon Delaware common stock as reported on the New York Stock Exchange on the last trading day prior to the effective time of the merger. All fractional interests to which a single holder would be entitled shall be aggregated and calculations shall be rounded to three decimal places. Any cash payment in exchange for a fractional interest will be made in U.S. dollars and will be rounded to the nearest cent; and



•

Each issued share of Aon Mergeco will be converted by operation of law into one share of Aon Delaware (as the surviving corporation), which will be held by Aon Intermediate.

44

---

Table of Contents

•

The Class A Ordinary Shares shall have been deemed eligible for deposit, book-entry and clearance services by DTC and its affiliates;



•

In addition, all shares of Aon Delaware common stock held in treasury immediately prior to the merger will automatically be cancelled and retired and will cease to exist, without consideration being delivered for such cancellation and all Class B Ordinary Shares of Aon UK will remain outstanding, but will have no voting rights or rights to dividends or distributions to the extent that they are held by Aon Delaware.



•

Each outstanding option, share of restricted stock and other equity-based award issued under our equity incentive plans for the purchase or receipt of, or benefits or amounts based on, shares of Aon Delaware common stock will represent the right to purchase or receive, or receive benefits or amounts based on, as applicable, the same number of Class A Ordinary Shares of Aon UK on the same terms.



•

Aon Mergeco or Aon Delaware as the surviving corporation will cause the Class A Ordinary Shares to be delivered to the Aon Delaware stockholders.

        The merger agreement may be amended, modified or supplemented at any time by the parties thereto before or after it is adopted by the stockholders of Aon Delaware. However, after adoption by the stockholders, no amendment, modification or supplement may be made or effected that requires further approval by Aon Delaware stockholders without obtaining that approval. Further, our Board may abandon the merger at any time, including after stockholder approval.

Ownership in Aon UK; Conversion of Shares; Delivery of Shares to Former Record Holders

        The conversion of Aon Delaware common stock into the right to receive the merger consideration will occur automatically at the effective time of the merger. The exchange agent will, as soon as reasonably practicable after the effective time of the merger, exchange certificates representing Aon Delaware shares for the merger consideration in the form of Class A Ordinary Shares to be received in the merger pursuant to the terms of the merger agreement, including the receipt of certificates and properly executed letter of transmittal, where applicable. The interests in the Class A Ordinary Shares will be contributed to Aon Mergeco prior to the effective time of the merger and, following the effective time of the merger, the Class A Ordinary Shares will be delivered to the exchange agent through the facilities of DTC for credit to Aon Delaware stockholders upon surrender of the certificates representing Aon Delaware common stock.

        If you are currently a beneficial holder of Aon Delaware common stock (i.e., your shares are held in "street name"), your ownership of Class A Ordinary Shares will be recorded in book entry form by your broker on the effective date of the merger.

        All other Class A Ordinary Shares delivered in connection with the merger will also initially be delivered through the facilities of DTC. If you hold share certificates representing Aon Delaware shares or if you otherwise hold Aon Delaware shares as record holder, your Class A Ordinary Shares will initially be delivered to the Custodian, as custodian for you. Any such shares will be evidenced by a statement issued by the Custodian. No former record holder of Aon Delaware whose Class A Ordinary Shares are delivered to the Custodian will be required to pay any fees to the Custodian for the initial 90 day period. Any such holder who wishes to transfer its shares from the custody of the Custodian to another bank or broker or to receive certificated Class A Ordinary Shares will not be charged any fees to do so by the Custodian or Aon UK.

        Following the effective time, such former record holders of Aon Delaware will be provided with terms and conditions governing their custody account and must provide the Custodian with "know your customer" information and other customary documentation and must agree to the terms and conditions if they wish the custodian to continue holding their Class A Ordinary Shares. If a former record holder

45

---

Table of Contents

does not provide such documentation within 90 days of the effective time of the merger, its shares will be debited from its custody account with the Custodian and it will be issued a certificate in respect of its Class A Ordinary Shares by the Company or its transfer agent or registrar, and it will subsequently hold your shares of Aon UK as a registered or record shareholder. Certain non-U.S. shareholders will not be eligible to hold their shares through the Custodian after the initial 90 day period. Any such shareholders who do not inform the Custodian of another bank or broker to whom their shares should be transferred prior to the end of the initial 90 day period will be issued shares in certificated form.

        Transfers of shares by record holders of the Company (including a transfer of shares to a broker-dealer or other nominee or a private transfer between individuals on the register) will generally attract stamp duty or stamp duty reserve tax, or "SDRT" under U.K. law. For more information, see "Material Tax Considerations Relating to the Merger—Material U.K. Tax Considerations Relating to and After the Merger—Stamp Duty and SDRT". As a result, if a former record holder of shares of Aon Delaware does not provide the Custodian with "know your customer" and other customary information in respect of shares delivered in connection with the merger within 90 days of the effective time, such a holder will be issued a certificate and will not be able to transfer those shares into DTC without incurring stamp duty and/or SDRT. As a result, each former record holder is strongly encouraged to provide the information requested by the Custodian in a timely manner, so the shares may remain within the facilities of DTC.

        If you hold Aon Delaware stock certificates, soon after the effective time of the merger, you will be sent a letter of transmittal, which is to be used to surrender your Aon Delaware stock certificates and to request that Class A Ordinary Shares be delivered to you. The letter of transmittal will contain instructions explaining the procedure for surrendering Aon Delaware stock certificates and receiving Class A Ordinary Shares, which shall initially be issued to the Custodian noted above in non-certificated book entry form. The letter of transmittal will specify that delivery shall be effected, and risk of loss and title to Aon Delaware stock certificates shall pass, only upon actual and proper delivery of the certificates to the exchange agent. Only those certificated holders of Aon Delaware common stock who properly surrender their common stock certificates in accordance with the instructions contained in the letter of transmittal will receive Class A Ordinary Shares and cash in lieu of any fractional interests. YOU SHOULD NOT RETURN STOCK CERTIFICATES WITH THE ENCLOSED PROXY CARD. If a certificate for Aon Delaware common stock has been lost, stolen or destroyed, the exchange agent will issue the merger consideration properly deliverable under the merger agreement upon compliance by the applicable stockholder with the replacement requirements established by the exchange agent. After the effective time, each certificate representing shares of Aon Delaware common stock that has not been surrendered will represent only the right to receive upon surrender of that certificate, the Class A Ordinary Shares and cash in lieu of fractional interests, if any, to which such holder is entitled. Following completion of the merger, Aon Delaware will not register any transfers of Aon Delaware common stock outstanding on its stock transfer books prior to the merger.

Dividends and Distributions; Withholding; Termination of Exchange Fund; No Liability

        Until Aon Delaware stock certificates or book-entry shares are surrendered for exchange, any dividends or other distributions of Aon UK declared after the effective time of the merger with respect to Class A Ordinary Shares will accrue but will not be paid. Aon UK will pay to former Aon Delaware stockholders any unpaid dividends or other distributions, without interest, only after they have duly surrendered their Aon Delaware stock certificates or book-entry shares. After the effective time of the merger, there will be no transfers on the stock transfer books of Aon Delaware of any Aon Delaware shares. If stock certificates representing Aon Delaware shares are presented for transfer after the completion of the merger, they will be cancelled and exchanged for the merger consideration into which the Aon Delaware shares represented by that certificate have been converted.

46

---

Table of Contents

        The exchange agent will be entitled to deduct and withhold from the merger consideration payable to any Aon Delaware stockholder the amounts it is required to deduct and withhold under the Code or any provision of any state, local or foreign tax law. If the exchange agent withholds any amounts, these amounts will be treated for all purposes of the merger as having been paid to the stockholders from whom they were withheld.

        Any portion of the securities or funds deposited with the exchange agent that remain undistributed to the former holders of shares of Aon Delaware common stock at the twelve month anniversary of the effective time of the merger will be delivered to Aon Delaware, as the surviving corporation, upon demand, or to its designee, and any former holder of shares of Aon Delaware common stock who has not theretofore properly surrendered its stock certificates shall thereafter look only to Aon Delaware, as the surviving corporation, for payment of any consideration due to it hereunder.

        None of Aon Delaware, Aon UK or the exchange agent will be liable to any former holder of shares of Aon Delaware common stock for any securities properly delivered or any amount properly paid by the exchange agent or its nominee, as the case may be, to a public official pursuant to applicable abandoned property, escheat or similar law nine months after the effective time of the merger. If any certificate representing shares of Aon Delaware common stock has not been surrendered prior to two years after the effective time of the merger (or immediately prior to an earlier date on which the Class A Ordinary Shares in respect of the stock certificate would otherwise escheat to or become the property of any governmental entity) any cash, share dividends and distributions otherwise payable in respect of the certificate shall, to the extent permitted by applicable law, become the property of Aon Delaware, as the surviving corporation, free and clear of all claims or interest of any person previously entitled thereto.

Subsequent Actions

        The merger is part of a reorganization of Aon Delaware's corporate structure. Following the consummation of the merger, Aon UK, as Aon Delaware's ultimate parent company, expects to examine various potential transactions that could bring about a further repositioning of the organizational structure of Aon Delaware and certain of its subsidiaries and their respective assets, in order to achieve the benefits of the reorganization described above under "—Background and Reasons for the Merger." We refer to these activities and transactions in this proxy statement/prospectus as "subsequent actions."

Possible Abandonment

        Pursuant to the merger agreement, the Board may exercise its discretion to terminate the merger agreement, and therefore abandon the reorganization, at any time, including after stockholder approval. Please see "Risk Factors—Our Board may choose to defer or abandon the merger at any time."

Additional Agreements and Actions

        The "Comparison of Rights of Stockholders/Shareholders—Indemnification of Directors and Officers" sets out the position under English law in relation to director and officer liability and indemnification. We expect to enter into new indemnity agreements and deeds of indemnity with directors, executive officers and certain other officers and employees (including directors, officers and employees of subsidiaries and other affiliates). These indemnification agreements and deeds of indemnity will require that Aon UK indemnify and hold harmless such persons, to the fullest extent permitted by applicable law, against all losses suffered or incurred by him or her which arise in connection with his or her appointment as a director or officer, as applicable, an act done in connection with his or her performance of his or her functions as a director or officer or an official investigation ordered into the affairs of the company of which he or she is serving as a director or

47

---

Table of Contents

officer at the request of the indemnifying company. However, the Companies Act prohibits an English company from indemnifying directors and certain officers in particular circumstances which are set out in the relevant indemnity agreements and generally summarized in the "Comparison of Rights of Stockholders/Shareholders—Indemnification of Directors and Officers."

        The reorganization does not constitute a change in control for purposes of Aon Delaware's equity incentive plans and change in control severance agreements. We have amended and/or intend to amend the equity incentive plans, change in control severance agreements and certain other plans and agreements, as necessary or advisable and to the extent permitted, to provide that any outstanding equity award is converted into rights to Class A Ordinary Shares on an equivalent one-share to one-share basis as of the effective time of the merger and to provide that the reorganization will not constitute a change in control under the terms of the applicable plans or agreements. Please see below "—Stock Compensation and Benefit Plans and Programs." Certain of our executive officers who will relocate to London following the merger had protection in their employment agreements that allowed them to terminate such agreements and receive severance benefits for "good reason" on account of their required relocation. Each executive officer that would have been entitled to such a termination in connection with the merger has consented to his or her international assignment and waived such severance protection in connection with his or her assignment and repatriation thereafter.

        In connection with the consummation of the reorganization, we do not expect any material actions to be required with respect to our (i) 8.205% junior subordinated deferrable interest debentures due January 2027, which have an aggregate principal amount outstanding of $687.0 million as of September 30, 2011, (ii) 6.25% €500 million debt securities due July 2014, which have an aggregate principal amount outstanding of €500 million as of September 30, 2011, (iii) 5.00% senior notes due September 2020, which have an aggregate principal amount outstanding of $600 million as of September 30, 2011, (iv) 3.50% senior notes due September 2015, which have an aggregate principal amount outstanding of $600 million as of September 30, 2011, (v) 5.05% CAD 375 million debt securities due April 2011, which have an aggregate principal amount outstanding of CAD 375 million as of September 30, 2011, (vi) 6.25% senior notes due September 2040, which have an aggregate principal amount outstanding of $300 million as of September 30, 2011, and (vii) 7.375% debt securities due December 2012, which have an aggregate principal amount outstanding of $225 million as of September 30, 2011. However, we intend to enter into supplemental indentures and other amendments as necessary or advisable to provide for guarantees by Aon UK or certain of its subsidiaries in connection with the reorganization. We will also provide financial information regarding Aon Delaware in the manner required by the aforementioned instruments. Refer to "Forward-Looking Statements" and "Risk Factors" above for a description of certain risks associated with the reorganization.

        We have a $450 million unsecured term loan facility (the "US Term Facility") with a syndicate of lenders, the proceeds of which were used to refinance a prior term loan facility. The US Term Facility matures in October 2013 and as of September 30, 2011, $438.8 million was outstanding under the US Term Facility.

        We also have a $400 million unsecured revolving credit facility (the "US Revolving Facility") and a €650 million unsecured revolving credit facility (the "Euro Revolving Facility"), with syndicates of lenders for general corporate purposes. The US Revolving Facility matures in December 2012 and the Euro Facility matures in October 2015. As of the date of this proxy statement/prospectus, there were no amounts outstanding under the US Revolving Facility and there were no amounts outstanding under the Euro Revolving Facility.

        In connection with the reorganization, we expect to seek the amendment or waiver of certain provisions of, or to renegotiate the terms of, the US Term Facility, the US Revolving Facility and the Euro Revolving Facility. If we are unsuccessful in modifying these credit facilities by the effective time of the merger, we may be in breach of certain provisions of these credit facilities.

48

---

Table of Contents

Conditions to Completion of the Merger

        Unless, among other things, the following conditions or requirements are satisfied or, if allowed by law, waived, the merger, and therefore the reorganization, will not be completed:

•

upon the SEC's declaration of effectiveness of the registration statement on Form S-4 with respect to the merger, Aon Delaware will have contributed all interests in the Class A Ordinary Shares to Aon Intermediate, which will in turn have contributed all such interests in the Class A Ordinary Shares to Aon Mergeco;



•

the merger agreement will have been adopted by the requisite vote of Aon Delaware stockholders;



•

Aon UK will have re-registered as a public limited company;



•

following stockholder approval of the merger agreement, Aon Delaware will have contributed all shares of Aon Intermediate to Aon UK;



•

a special resolution will have been passed to alter the rights of the holders of the Class A Ordinary Shares to have the same rights as holders of Class B Ordinary Shares, nominal value £0.10 each, or "Class B Ordinary Shares," conditional upon the merger becoming unconditional;



•

the Class A Ordinary Shares will have been authorized for listing on the NYSE, subject to official notice of issuance and satisfaction of other standard conditions;



•

the Class A Ordinary Shares shall have been deemed eligible for deposit, book-entry and clearance services by The Depository Trust Company and its affiliates;



•

McDermott Will & Emery LLP, legal counsel to Aon Delaware, will have delivered a tax opinion in form and substance acceptable to Aon Delaware;



•

no decree, order or injunction will have been in effect with respect to the SEC registration statement of which this proxy statement/prospectus is a part;



•

other than the filing of the certificate of merger with the Secretary of State of Delaware in connection with the merger, all material consents and authorizations of, filings or registrations with, and notices to, any governmental or regulatory authority required of Aon Delaware, Aon Mergeco or any of their respective subsidiaries to consummate the merger and the other transactions contemplated by the merger agreement, including without limitation any consents, authorizations, filings or registrations required under (i) applicable U.S. state securities and "Blue Sky" laws, (ii) U.K. financial services laws, and (iii) laws governing insurance brokerage in various countries in which we do business, shall have been obtained or made; and



•

none of the parties to the merger agreement shall be subject to any decree, order or injunction of any court of competent jurisdiction, whether in the U.S., the U.K., the European Union or any other country, that prohibits the consummation of the merger.

        Our Board does not intend to waive (where capable of waiver) any of these conditions or requirements unless it determines that the merger is in the best interests of Aon Delaware and our stockholders, despite the condition(s) not being satisfied in whole or in part. Notwithstanding the foregoing, our Board reserves the right to defer or abandon the merger at any time, including after stockholder approval, for the reasons described under "Risk Factors—Our Board may choose to defer or abandon the merger at any time."

        Approval of the reduction of capital of Aon UK and the approval of the relocation compensation in the advisory, non-binding vote described in this proxy statement/prospectus are not conditions to the completion of the merger.

49

---

Table of Contents

Stock Compensation and Benefit Plans and Programs

        We have a variety of equity incentive plans, compensation plans, and other plans, agreements, awards and arrangements outstanding that provide for options, restricted stock units, performance share units or other rights to purchase or receive shares of Aon Delaware (or the right to receive benefits or amounts by reference to those shares). We refer to these plans, agreements, awards and arrangements as our equity incentive plans. As of the effective time of the merger, Aon UK will adopt and assume, and become the plan sponsor of, the Aon Stock Incentive Plan, the Amended and Restated Global Stock and Incentive Plan of Hewitt Associates, Inc., the Aon Corporation 2011 Incentive Plan, and each change-in-control agreement with a key employee and under the Aon Corporation Executive Special Severance Plan, each as may be amended from time to time. With respect to all other existing plan and arrangements, Aon Delaware will remain the plan sponsor. With respect to all equity incentive plans or arrangements, however, Class A Ordinary Shares, rights to Class A Ordinary Shares or benefits or amounts based on Class A Ordinary Shares will thereafter be granted or issued under each equity incentive plan instead of shares of Aon Delaware, rights to shares of Aon Delaware or benefits or amounts based on shares of Aon Delaware currently being granted or issued under such plans. Future awards would be subject to and governed by the terms of our equity incentive plans, as assumed by Aon UK if applicable, and any agreements entered into pursuant thereto.

        The equity incentive plans and certain other plans and agreements have been and/or will be amended or modified (i) to facilitate the assumption and adoption by Aon UK of certain equity incentive plans and the various rights, duties or obligations thereunder, as described in this proxy statement/prospectus, (ii) to the extent required to reflect the issuance of rights over Class A Ordinary Shares (rather than rights over Aon Delaware shares), (iii) to provide for the appropriate substitution of Aon UK in place of Aon Delaware where applicable, (iv) to provide that the merger will not constitute a change in control under the terms of the equity incentive plans, change in control severance agreements and certain other plans and agreements to the extent permitted or applicable, and (v) to comply with applicable English or U.S. corporate or tax law requirements. Stockholder approval of the adoption of the merger agreement will also constitute stockholder approval of the adoption and assumption of the equity incentive plans by Aon UK as contemplated by this proxy statement/prospectus. Equity incentive plans that provide benefits to employees and directors of Aon Delaware and its subsidiaries will, if applicable, upon being assumed by Aon UK, continue to provide benefits to such employees or directors consistent with the current manner, subject to the limitations set forth in the relevant plan, as such plan may be amended from time to time. To the extent that awards of restricted stock or similar restricted share rights are granted following the effective time of the merger, the holder may be obligated to pay the nominal or par value to receive the Class A Ordinary Shares due to English corporate law requirements.

        At the effective time of the merger, all outstanding options to purchase shares of Aon Delaware common stock granted or issued prior to the effective time of the merger, and all outstanding restricted stock units of Aon Delaware and other equity-based awards awarded under our equity incentive plans, will entitle the holder to purchase or receive, or receive benefits or amounts based on, as applicable, an equal number of Class A Ordinary Shares. All of such stock options, restricted stock units, performance share units and other equity-based awards will be subject to the same terms and conditions as were applicable to such awards granted or issued by Aon Delaware immediately prior to the effective time of the merger other than as discussed above. However, the Companies Act prohibits an English company from issuing shares for no consideration, including with respect to grants of restricted stock units or performance share units made pursuant to equity incentive plans. Accordingly, the nominal value of the shares issued upon the lapse of restrictions or the vesting of any share-based grant underlying any Class A Ordinary Shares must be paid pursuant to the Companies Act.

50

---

Table of Contents

        Holders of outstanding nonqualified options, awards of restricted stock or other equity-based awards, as well as beneficial owners of Aon Delaware shares acquired under the Aon Savings Plan, other than U.S. taxpayers who perform all of their services within the U.S., may be subject to tax as a result of the conversion of the underlying shares of Aon Delaware common stock to Class A Ordinary Shares, depending on the country where the holders are citizens or tax residents or the country where they resided during the life of such equity awards of Aon Delaware. However, holders of outstanding nonqualified options and beneficial owners of Aon Delaware shares acquired under the Aon Savings Plan who are U.S. taxpayers should not recognize taxable gain for U.S. income tax purposes as a result of the conversion. Tax withholding and/or reporting may be required by the Company or one of its affiliates and/or the holder of the applicable equity award, and certain employer social insurance contributions or other taxes may be due as a result of the conversion of the equity awards from Aon Delaware awards to Class A Ordinary Shares awards. Depending on the country where the holders are citizens or residents or the country where they resided during the life of the Aon Delaware awards, it is also possible that the conversion of equity awards will trigger certain regulatory filings or notices to employees concerning the tax consequences.

Interests of Directors and Executive Officers in the Merger

        The directors and executive officers of Aon Delaware have interests in the transaction that are in addition to their interests as stockholders of Aon Delaware generally. Our Board was aware of these interests and considered them, among other matters, in approving the merger and in determining to recommend that Aon Delaware stockholders adopt the merger agreement.

Executive Officers of Aon Delaware

        In connection with the proposed relocation of the headquarters of the Company to London, England, the Organization and Compensation Committee of the board of directors of the Company (the "Committee"), after consulting with its independent executive compensation consultant, Frederic W. Cook & Co., Inc., approved relocation benefits for those executive officers who will relocate to the new corporate headquarters in London, including Gregory C. Case (the Company's President and Chief Executive Officer), Christa Davies (the Company's Executive Vice President and Chief Financial Officer), Stephen P. McGill (Chairman and Chief Executive Officer of Aon Risk Solutions) and Michael J. O'Connor (Chief Operating Officer of Aon Risk Solutions).

        In connection therewith, each of our named executive officers (other than Baljit Dail, the Co-Chief Executive Officer of Aon Hewitt, who will not relocate to London) and certain other senior executives entered into an international assignment letter with the Company. The letter describes the international assignment and sets forth the relocation benefits available to the executive, which are described below. The letter is not intended to diminish the rights of the executive under his or her current employment arrangement; however, the letter provides by its terms that the executive's acceptance of the international assignment, and repatriation thereafter, shall not give rise to any right to terminate for good reason (as such term is defined in the executive's employment agreement, if applicable). The potential cost had such protection been triggered by the executives' resignation for good reason would have been approximately $60 million. The letter will only remain in effect during the international assignment, and is not intended to continue for more than two years.

        Relocation benefits are customary for expatriate assignments for the Company and other employers in its industry. The relocation packages approved are intended to: keep the transferred executives "whole" on a total rewards basis; be transparent and equitable; and reflect best practices and benchmarks of industry counterparts. The Committee will periodically review the relocation packages of the Company's executive officers.

51

---

Table of Contents

        Depending on each executive's personal circumstances, and as disclosed in the chart and the related footnotes herein, the relocation packages generally provide some or all of the following benefits, each shown in U.S. dollars:

•

A monthly housing allowance of approximately (a) $28,000 for Mr. Case and (b) $21,000 for the other executive officers;



•

A monthly cost of living differential, equal to 15% of the executive's monthly base salary (such monthly salary considered in the formula is subject to a cap of $50,000);



•

A monthly foreign service allowance of up to the monthly equivalent of 15% of the executive's annual base salary as of January 12, 2012;



•

An annual waiver/retention bonus, ranging from $0 for Mr. Case to $250,000 for certain of the other relocating executive officers;



•

A monthly transportation allowance of approximately $2,000;



•

An annual home leave benefit of $5,000 per family member, providing one round-trip air fare for the executive, his or her partner and eligible dependents;



•

Eligible dependents' schooling assistance, including tuition and application fees;



•

A households goods move at the beginning and end of the international assignment;



•

A one-time relocation allowance of the sum of the executive's monthly base salary plus $10,000 (capped at $80,000);



•

A tax equalization benefit (without a tax gross-up feature insulating the executive from incurring any U.S. income or social tax, such as FICA, on the relocation benefits) designed to equalize the income tax paid by the executive so that his or her total income and social tax costs related to any earnings from the Company while on the international assignment (including earnings related to granting or vesting of equity-based awards) will be no more than an amount the executive would have paid had all of the earnings been taxable solely pursuant to the U.S. income and social tax laws; and



•

Enhanced tax preparation and planning and expatriate services for the tax years covered by the international assignment or for which international earnings are taxed by the host country.

        The tax equalization benefit will not protect the executives in their capacity as shareholders of the Company from capital gains recognized pursuant to U.S. federal income tax law as a result of the merger. The tax equalization benefit will cap the executive's total income and social tax exposure to that he or she would be taxed on earnings from the Company under the U.S. tax laws (as compared to the U.K. tax laws as in existence from time to time). The benefit will not include a gross-up for U.S. income and social taxes on any earnings, including any taxable relocation benefits.

        The annual waiver/retention bonus is subject to recoupment under certain circumstances resulting in termination of the executive officer's employment during the period of the international assignment. All of the relocation benefits are subject to recoupment if the executive officer resigns employment with the Company within two years of commencing the international assignment, or twelve months after the end thereof, and becomes employed by a direct competitor of the Company.

        Effective January 12, 2012, the Committee also approved a change to the annual bonus eligibility for each of the relocating executive officers other than Mr. Case, such that for performance in 2012 and 2013 the target bonus will be a percentage of the sum of the executive's base salary plus his or her foreign service allowance. Previously, the target bonus was a percentage of base salary only. The target percentages remain unchanged (i.e. Ms. Davies' is 150%; Mr. McGill's is 175%; and Mr. O'Connor's is 100%).

52

---

Table of Contents

        Mr. Case will be based in London and will secure a residence in London. To alleviate the additional expense and inconvenience to be incurred by Mr. Case and his family, the Committee, following consultation with and concurrence by the other independent directors, approved a housing allowance, relocation allowance, foreign service allowance and cost of living differential for Mr. Case. There will be no tax equalization on the allowances. No other benefits related to the relocation of the Company's headquarters will be provided to Mr. Case.

        The following executive officers of the Company (who are not named executive officers) will also relocate to London and will be provided the two-year relocation benefits described above, with the cost estimated in the aggregate: Peter Lieb ($1,555,123); Gregory Besio ($1,554,044); and Philip Clement ($1,636,119).

        With respect to the Company's named executive officers (other than Mr. Dail), the relocation package will contain the following benefits for the two-year term (shown in U.S. dollars) in addition to the executives' current compensation arrangements:

"Golden Parachute" Compensation

53

---

Table of Contents

54

---

Table of Contents

Directors of Aon Delaware

        In connection with the merger, Aon UK intends to enter into customary indemnification agreements with the directors and officers of Aon UK. For more information, see "—Additional Agreements and Actions."

Effective Time of the Merger

        We anticipate that the merger will be completed in the second quarter of 2012. Our Board will have the right, however, to defer or abandon the merger at any time, including after stockholder approval.

Management of Aon UK

        Immediately prior to the effective time of the merger, the directors and executive officers of Aon UK will be the same as the then-current directors and executive officers of Aon Delaware, and the directors of Aon Delaware will be the directors of Aon UK. If the merger is completed, the members of the Aon UK board of directors, or the "Aon UK Board," will serve until the earlier of the next meeting of shareholders at which an election of directors is required and until their successors are elected, and the executive officers will serve until their successors are elected or earlier death, disability or retirement.

55

---

Table of Contents

Recommendation and Required Vote

        The affirmative vote of the holders of at least two-thirds of the outstanding shares of Aon Delaware common stock entitled to vote at the special meeting is required to approve the proposal to adopt the merger agreement. Our Board believes that the reorganization, to be effected by the merger agreement and any subsequent actions, is advisable and in the best interests of Aon Delaware and our stockholders. ACCORDINGLY, OUR BOARD HAS APPROVED THE REORGANIZATION AND THE MERGER AGREEMENT. OUR BOARD RECOMMENDS THAT STOCKHOLDERS VOTE "FOR" THE ADOPTION OF THE MERGER AGREEMENT.

Regulatory Approvals

        The only governmental or regulatory approvals or actions that are required to complete the merger are compliance with U.S. federal and state securities laws, the NYSE rules and regulations, U.K. financial services laws, laws governing insurance brokerage in various countries in which we do business, and the DGCL (including the filing with the Secretary of State of the State of Delaware of a certificate of merger).

Rights of Dissenting Stockholders

        Under the DGCL, you will not have appraisal rights in connection with the merger because, among other reasons, the Class A Ordinary Shares you have a right to receive in the merger will be listed on the NYSE.

Comparison of Rights of Holders of Aon Delaware Shares with Holders of Class A Ordinary Shares

        There are numerous differences between the rights of a stockholder in Aon Delaware, a Delaware corporation, and the rights of a holder of Class A Ordinary Shares of Aon UK, a company incorporated under English law. The characteristics of and the differences between Aon Delaware common stock and the Class A Ordinary Shares are summarized under ""Description of the Class A Ordinary Shares of Aon UK" and "Comparison of Rights of Stockholders/Shareholders."

Stock Exchange Listing

        Aon Delaware common stock is currently listed on the NYSE under the symbol "AON." There is currently no established public trading market for the Class A Ordinary Shares. However, it is a condition to the completion of the merger that the Class A Ordinary Shares will be authorized for listing on the NYSE, subject to official notice of issuance and satisfaction of other standard conditions. As such, we expect that as of the effective time of the merger, the Class A Ordinary Shares will be authorized for listing on the NYSE, and we expect such Class A Ordinary Shares will be listed on the NYSE under the symbol "AON."

56

---

Table of Contents

Dividends

        The following table sets forth the quarterly cash dividends paid per share of Aon Delaware common stock in the periods indicated.

        On January 11, 2012, Aon Delaware announced that its Board had declared a dividend of $0.15 per share to be paid on February 15, 2012, to shareholders of record on February 1, 2012. The Aon UK Board currently intends to declare quarterly dividends out of current earnings, if appropriate in view of its profitability, liquidity, financial condition, reinvestment opportunities and capital requirements. Aon UK will initially adopt Aon Delaware's prior practice with respect to dividends, which has been to pay quarterly cash dividends on our common stock since the first quarter of 1999, subject to the discretion of the board of directors. Following completion of the merger, as part of an overall review of capital allocation activities, Aon UK will evaluate both its dividend and share repurchase policies. We expect that Aon UK would make quarterly dividends after the completion of this review. Notwithstanding the foregoing, the declaration and payment of all future dividends and any share repurchases will remain subject to the discretion of Aon UK's Board and the limitations set forth in the Companies Act, which generally provide that dividends, distributions and share repurchases may only be made out of distributable reserves. "See Risk Factors—English law will require that we meet certain additional financial requirements before we declare dividends and repurchase shares following the merger."

Accounting Treatment of the Merger

        Historical cost basis accounting, rather than acquisition method accounting, will be applied to the merger under U.S. GAAP. Given that, for accounting purposes, there will be no change in control and Aon Delaware's stockholders will be in the same economic position immediately before and after the reorganization, and because the reorganization will be accounted for as an internal reorganization of entities under common control, there will be no revaluation of Aon Delaware's assets and liabilities.

Effect of the Reorganization on Potential Future Status as a Foreign Private Issuer

        We do not believe that, after the completion of the reorganization, Aon UK will be classified as a "foreign private issuer" within the meaning of the rules promulgated under the Exchange Act. The definition of a "foreign private issuer" has two parts—one based on a company's percentage of U.S. resident shareholders and the other on its business contacts with the U.S. An organization incorporated

57

---

Table of Contents

under the laws of a country outside the U.S. qualifies as a foreign private issuer unless both parts of the definition are satisfied as of the last business day of its most recently completed second fiscal quarter.

        The shareholder test requires more than 50 percent of the outstanding voting securities of a non-U.S. domiciled organization to be held by U.S. residents. We currently expect that Aon UK will meet the shareholder test upon the completion of the reorganization because we understand that more than 50 percent of our outstanding voting securities are held by U.S. residents.

        The business contacts test requires that any of the following be true with respect to a non-U.S. domiciled organization: (i) the majority of its executive officers or directors are U.S. citizens or residents; (ii) more than 50 percent of its assets are located in the U.S.; or (iii) its business is administered principally in the U.S. Because it is anticipated that a majority of Aon UK's directors immediately following the merger will be U.S. citizens or residents, we currently expect that Aon UK will meet the business contacts test upon the completion of the reorganization. However, at some time in the future, Aon UK could qualify for status as a foreign private issuer. If that occurs, Aon UK would be exempt from certain requirements applicable to U.S. public companies, including, among others:

•

the rules requiring the filing of Quarterly Reports on Form 10-Q and Current Reports on Form 8-K with the SEC;



•

the SEC's rules regulating proxy solicitations;



•

the provisions of Regulation FD;



•

the filing of reports of beneficial ownership under Section 16 of the Exchange Act (although beneficial ownership reports may be required under Section 13 of the Exchange Act); and



•

"short-swing" trading liability imposed on insiders who purchase and sell securities within a six-month period.

58

---

Table of Contents

MATERIAL TAX CONSIDERATIONS RELATING TO THE MERGER

        This section contains a general discussion of certain material tax consequences of (1) the merger, (2) post-merger ownership and disposition of Aon UK shares and (3) post-merger operations of Aon UK.

        The discussion under the caption "—U.S. Federal Income Tax Considerations" addresses (1) application of the U.S. anti-inversion rules to Aon Delaware, Aon UK and the exchanging holders, (2) the material U.S. federal income tax consequences of the merger to Aon Delaware and to Aon UK, and (3) the material U.S. federal income tax consequences to U.S. holders and non-U.S. holders (each as defined below) of (a) exchanging Aon Delaware shares for Aon UK shares in the merger and (b) owning and disposing of Aon UK shares received in the merger.

        The discussion of the merger and of ownership and disposition of Aon UK shares received in the merger under the caption "—U.K. Tax Considerations" addresses the material U.K. tax consequences to stockholders of Aon Delaware resident for tax purposes in the U.K. and in a country other than the U.K. It also addresses certain U.K. tax considerations of the merger and subsequent operations for Aon UK and Aon Delaware.

        The discussion below is not a substitute for an individual analysis of the tax consequences of the merger, post-merger ownership and disposition of Aon UK shares or post-merger operations of Aon UK. You should consult your own tax advisor regarding the particular U.S. (federal, state and local) and U.K. and other non-U.S. tax consequences of these matters in light of your particular situation.

U.S. Federal Income Tax Considerations

Scope of Discussion

        This discussion generally does not address any aspects of U.S. taxation other than U.S. federal income taxation, is not a complete analysis or listing of all potential tax consequences of the merger or of holding and disposing of Aon UK shares, and does not address all tax considerations that may be relevant to Aon Delaware shareholders. In particular, the discussion below addresses tax consequences to those who hold their Aon Delaware shares, and who will hold their Aon UK shares, solely as capital assets. The discussion below does not address any tax consequences to Aon Delaware or Aon UK stockholders or shareholders, as applicable, who, for U.S. federal tax purposes, are subject to special rules, such as:

•

banks, financial institutions or insurance companies;



•

tax-exempt entities;



•

persons who hold shares as part of a straddle, hedge, integrated transaction or conversion transaction;



•

persons who have been, but are no longer, citizens or residents of the U.S.;



•

persons holding shares through a partnership or other fiscally transparent person;



•

dealers or traders in securities, commodities or currencies;



•

grantor trusts;



•

persons subject to the alternative minimum tax;



•

U.S. persons whose "functional currency" is not the U.S. dollar;



•

regulated investment companies and real estate investment trusts;

59

---

Table of Contents

•

persons who received the Aon Delaware shares through the exercise of incentive stock options or through the issuance of restricted stock under an equity incentive plan or through a tax qualified retirement plan; or



•

persons who own (directly or through attribution) 10 percent or more, determined by voting power or value, of Aon Delaware common stock or Aon UK common stock.

        This discussion is based on the Code, the regulations promulgated thereunder by the U.S. Department of the Treasury, or the "Treasury Regulations," judicial and administrative interpretations thereof and the Convention Between the United States of America and the United Kingdom for the Avoidance of Double Taxation with respect to Taxes on Income, or the "U.K.-U.S. Tax Treaty," in each case as in effect and available on the date of this proxy statement/prospectus. Each of the foregoing is subject to change, potentially with retroactive effect, and any such change could affect the tax consequences described in this proxy statement/prospectus. It is expected that neither Aon Delaware nor Aon UK will request a ruling from the IRS as to the U.S. federal tax consequences of the merger, post-merger ownership and disposition of Aon UK shares or any other matter. There can be no assurance that the IRS will not challenge any of the U.S. federal tax consequences described below or, if challenged, will be sustained by a court.

        For purposes of this discussion, a "U.S. holder" is a beneficial owner of Aon Delaware shares or, after the completion of the merger, Aon UK shares, that for U.S. federal income tax purposes is:

•

an individual citizen or resident alien of the U.S.;



•

a corporation or other entity taxable as a corporation created or organized in or under the laws of the U.S. or any state thereof or the District of Columbia;



•

an estate the income of which is subject to U.S. federal income taxation regardless of its source; or



•

a trust, if such trust validly has elected to be treated as a U.S. person for U.S. federal income tax purposes or if (1) a U.S. court can exercise primary supervision over its administration and (2) one or more U.S. persons have the authority to control all of the substantial decisions of such trust.

        A "non-U.S. holder" is a beneficial owner of Aon Delaware shares or, after the completion of the merger, Aon UK shares, other than a U.S. holder or an entity or arrangement treated as a partnership for U.S. federal income tax purposes (a "Partnership"). If a Partnership is a beneficial owner of Aon Delaware shares or Aon UK shares, the tax treatment of a partner in that Partnership will generally depend on the status of the partner and the activities of the Partnership. Holders of Aon Delaware shares or Aon UK shares that are Partnerships and partners in such Partnerships should consult their tax advisors regarding the U.S. federal income tax consequences to them of the merger and the ownership and disposition of Aon UK shares. For purposes of this discussion, "holder" or "shareholder" means either a U.S. holder or a non-U.S. holder or both, as the context may require.

        For U.S. federal income tax purposes, the conversion of each issued and outstanding share of the common stock of Aon Delaware into an Aon UK share upon the consummation of the merger generally will be treated as an exchange of such share for the Aon UK share. Therefore, the U.S. federal income tax discussion contained herein refers to an "exchange" rather than a "conversion" in describing the U.S. federal income tax consequences of the merger.

The U.S. Anti-Inversion Rules

        Although Aon UK is, or will be, incorporated in the U.K., the IRS may assert that Aon UK should be treated as a U.S. corporation (and, therefore, a U.S. tax resident) for U.S. federal tax purposes under Section 7874 of the Code. At the time of enactment of Section 7874 in 2004, a number of

60

---

Table of Contents

publicly-traded U.S. multinational corporations had expatriated to non-U.S. jurisdictions. In most cases, those corporations expatriated to tax haven jurisdictions in which the applicable U.S. multinational corporation had no (or minimal) historic business activities. As a general matter, absent the application of Section 7874, a corporation is considered, for U.S. federal tax purposes, to be a tax resident of the jurisdiction in which it is incorporated.

        Under Section 7874, a corporation created or organized outside the U.S. will be treated as a U.S. corporation for U.S. federal tax purposes, when (i) the foreign corporation directly or indirectly acquires substantially all of the assets held directly or indirectly by a U.S. corporation, (ii) the former shareholders of the acquired U.S. corporation hold at least 80 percent of the vote or value of the shares of the foreign acquiring corporation by reason of holding stock in the U.S. acquired corporation, and (iii) the foreign corporation's "expanded affiliated group" does not have substantial business activities in the foreign corporation's country of incorporation relative to its expanded affiliated group's worldwide activities. Solely for purposes of Section 7874, "expanded affiliated group" means the foreign corporation and all subsidiaries in which the foreign corporation, directly or indirectly, owns more than 50 percent of the stock by vote and value.

        Aon UK will indirectly acquire all of the assets of Aon Delaware, and the former shareholders of Aon Delaware will acquire, as a result of the merger, 100 percent of the stock in Aon UK by reason of holding stock in Aon Delaware. After the merger, however, the Aon UK expanded affiliated group intends to continue the substantial business activities in the U.K. that the Aon Delaware expanded affiliated group currently conducts. Therefore, Aon UK should not be treated as a U.S. corporation for U.S. federal tax purposes.

        Section 7874 does not define the term "substantial business activities" or otherwise quantify the activities that the foreign corporation and its expanded affiliated group should have in the foreign corporation's country of incorporation. Temporary Treasury Regulations issued under Section 7874 of the Code in June 2009, or the "Applicable Regulations," also decline to define or quantify "substantial business activities." Rather, the Applicable Regulations provide that whether a foreign corporation's expanded affiliated group has substantial business activities in the foreign corporation's country of organization in relation to its worldwide activities is based on all facts and circumstances. The Applicable Regulations provide that relevant items to be considered in determining whether an expanded affiliated group has substantial business activities in a foreign country when compared to the total activities of the expanded affiliated group include, but are not limited to, the following:

(i)

the historical conduct of continuous business activities in the foreign country by the expanded affiliated group;



(ii)

the conduct of continuous business activities in the foreign country by the expanded affiliated group in the ordinary course of one or more active trades or businesses, involving:



(A)

property located in the foreign country that is owned by members of the expanded affiliated group,



(B)

the performance of services in the foreign country by employees of the expanded affiliated group, and



(C)

sales of goods to customers;



(iii)

the performance in the foreign country of substantial managerial activities by officers and employees of the expanded affiliated group who are based in the foreign country;



(iv)

a substantial degree of ownership of the expanded affiliated group by investors resident in the foreign country; and

61

---

Table of Contents

(v)

business activities in the foreign country that are material to the achievement of the overall business objectives of the expanded affiliated group.

        The Applicable Regulations further provide that the presence or absence of any of the above items is not determinative, and the weight given to any item depends on the facts and circumstances. The Applicable Regulations contain no safe harbor or examples to illustrate the application of the relevant factors to determine whether substantial business activities exist. There is no judicial or further administrative guidance on the meaning of "substantial business activities" for purposes of Section 7874.

        Notwithstanding the absence of a safe harbor or other quantitative guidance in the Applicable Regulations, we believe that the activities the Aon Delaware expanded affiliated group conducts in the U.K. should satisfy the substantial business activities test set forth in the Applicable Regulations. Moreover, it is a condition to closing of the merger that Aon Delaware receives from McDermott Will & Emery LLP, counsel to the Company, or "Counsel," an opinion of Counsel concluding that Section 7874 should not apply to the merger. Pending receipt of Counsel's opinion, our belief that Section 7874 should not apply to the merger is based on an analysis of the relevant items listed in the Applicable Regulations.

Material Tax Consequences of the Merger to Aon Delaware and Aon UK

        The merger should not give rise to material U.S. federal income tax for Aon Delaware or Aon UK.

Material Tax Consequences to Stockholders

Material Tax Consequences to U.S. Holders

Consequences of the Merger

        The merger should qualify as a "reorganization" within the meaning of Section 368(a) of the Code and/or an exchange under Section 351 of the Code. However, pursuant to special rules contained in Section 367(a) of the Code and the Treasury Regulations promulgated thereunder, U.S. holders exchanging their Aon Delaware shares for Aon UK shares pursuant to the merger should recognize gain, if any, but not loss. In general, for U.S. federal income tax purposes, a U.S. holder should recognize gain equal to the excess of the fair market value of the Aon UK shares received by the U.S. holder pursuant to the merger over such holder's adjusted basis in the Aon Delaware shares exchanged therefor. Any such gain should be capital gain, and should be long-term capital gain if the Aon Delaware shares have been held by such holder for more than one year at the effective time of the merger. A U.S. holder that recognizes gain pursuant to the merger should have an adjusted tax basis in the Aon UK shares it receives equal to the adjusted tax basis of the Aon Delaware shares exchanged therefor, increased by the amount of gain recognized.

        A U.S. holder should not be permitted to recognize any loss realized on the exchange of its Aon Delaware shares in the merger. The adjusted tax basis of the Aon UK shares received by a U.S. holder with a loss on its Aon Delaware shares should be equal to such U.S. holder's adjusted tax basis in its Aon Delaware shares surrendered in exchange therefor.

        In determining the amount of gain recognized, each of the Aon Delaware shares transferred should be treated as the subject of a separate exchange. Thus, if a U.S. holder transfers some Aon Delaware shares on which gains are realized and other Aon Delaware shares on which losses are realized, the U.S. holder may not net the losses against the gains to determine the amount of gain recognized. The holding period for any Aon UK shares received by U.S. holders should include the holding period of the Aon Delaware shares exchanged therefor.

62

---

Table of Contents

        Cash received by a U.S. holder in exchange for a fractional share of Aon Delaware common stock should be treated as having been received in redemption of such fractional share. Thus, gain or loss generally should be recognized by such U.S. holder in an amount equal to the difference between the amount of cash received and the U.S. holder's adjusted tax basis in such fractional share.

Receiving Distributions on Aon UK shares

        Subject to the discussion below under "—Passive Foreign Investment Company Provisions," U.S. holders will be required to include in gross income the gross amount of any distribution received on the Aon UK shares to the extent that the distribution is paid out of Aon UK's current or accumulated earnings and profits as determined for U.S. federal income tax purposes. We refer to such a distribution herein as a dividend. With respect to non-corporate U.S. holders, certain dividends received in taxable years beginning before January 1, 2013 from a qualified foreign corporation will be subject to U.S. federal income tax at a maximum rate of 15 percent. As long as the Aon UK shares are listed on the NYSE (or certain other stock exchanges) and/or Aon UK qualifies for benefits under the U.K.-U.S. Tax Treaty, Aon UK will be treated as a qualified foreign corporation for this purpose. This reduced rate will not be available in all situations, and U.S. holders should consult their own tax advisors regarding the application of the relevant rules to their particular circumstances. U.S. holders generally will not be eligible for dividends-received deduction with respect to dividends from Aon UK.

        Distributions in excess of the current and accumulated earnings and profits of Aon UK should be applied first to reduce the U.S. holder's tax basis in its Aon UK shares, and thereafter should constitute gain from the sale or exchange of such shares. In the case of a non-corporate U.S. holder, the maximum U.S. federal income tax rate applicable to such "gain" is 15 percent under current law if the holder's holding period for such Aon UK shares exceeds twelve months. This reduced rate is scheduled to expire effective for taxable years beginning after December 31, 2012. Special rules not described herein may apply to U.S. holders who do not have a uniform tax basis and holding period in all of their Aon UK shares, and any such U.S. holders are urged to consult their own tax advisors with regard to such rules.

Dispositions of Aon UK shares

        Subject to the discussion below under "—Passive Foreign Investment Company Provisions," a U.S. holder of Aon UK shares generally should recognize capital gain or loss for U.S. federal income tax purposes on the sale, exchange or other taxable disposition of Aon UK shares in an amount equal to the difference between the amount realized from such sale, exchange or other taxable disposition and the U.S. holders' tax basis in such shares. In the case of a non-corporate U.S. holder, the maximum U.S. federal income tax rate applicable to such gain is 15 percent under current law if the holder's holding period for such Aon UK shares exceeds twelve months. This reduced rate is scheduled to expire effective for taxable years beginning after December 31, 2012. The deductibility of capital losses are subject to limitations.

Passive Foreign Investment Company Provisions

        The treatment of U.S. holders of Aon UK shares in some cases could be materially different from that described above if, at any relevant time, Aon UK were a passive foreign investment company, which we refer to as a PFIC.

        For U.S. tax purposes, a foreign corporation is classified as a PFIC for any taxable year if either (1) 75 percent or more of its gross income is "passive income" (as defined for U.S. federal income tax purposes) or (2) the average percentage of assets held by such corporation which produce passive income or which are held for the production of passive income is at least 50 percent. For purposes of applying the tests in the preceding sentence, the foreign corporation is deemed to own its

63

---

Table of Contents

proportionate share of the assets, and to receive directly its proportionate share of the income, of any other corporation of which the foreign corporation owns, directly or indirectly, at least 25 percent by value of the stock. Aon UK believes that it will not be a PFIC following the merger.

        The tests for determining PFIC status are applied annually, and it is difficult to accurately predict future income and assets relevant to this determination. Accordingly, Aon UK cannot assure U.S. holders that it will not become a PFIC. If Aon UK should determine in the future that it is a PFIC, it will endeavor to so notify U.S. holders of Aon UK shares, although there can be no assurance that it will be able to do so in a timely and complete manner. U.S. holders of Aon UK shares should consult their own tax advisors about the PFIC rules, including the availability of certain elections.

Material Tax Consequences to Non-U.S. Holders

Consequences of the Merger

        A non-U.S. holder generally will not be subject to U.S. federal income or withholding tax on gain realized, if any, on the exchange of Aon Delaware shares for Aon UK shares or on the receipt of cash in exchange for fractional shares of Aon Delaware common stock unless: (1) such gain is effectively connected with the conduct by the non-U.S. holder of a trade or business within the U.S. (or, if a tax treaty applies, is attributable to a permanent establishment or fixed place of business maintained by the non-U.S. holder in the U.S.); (2) in the case of certain capital gains recognized by a non-U.S. holder that is an individual, such individual is present in the U.S. for 183 days or more during the taxable year in which the capital gain is recognized and certain other conditions are met; (3) the non-U.S. holder is subject to backup withholding; or (4) Aon Delaware is or has been a "U.S. real property holding corporation" within the meaning of Section 897(c)(2) of the Code at any time within the shorter of the five-year period preceding the merger or such non-U.S. holder's holding period, and the non-U.S. holder holds, or has held at any time during such shorter period, more than 5 percent of the Aon Delaware common stock. Aon Delaware does not believe that it is or has been a U.S. real property holding corporation within the last five years.

        If a non-U.S. holder is a citizen or resident of, or otherwise subject to taxation in, a country other than the U.S. or the U.K., the tax consequences of the exchange of Aon Delaware shares for Aon UK shares will depend on the applicable tax laws in such country.

Consequences of Owning and Disposing of Aon UK shares

        A non-U.S. holder generally should not be subject to U.S. federal income or withholding tax on dividends from Aon UK unless: (1) the dividends are effectively connected with the non-U.S. holder's conduct of a trade or business in the U.S. (or, if a tax treaty applies, the dividends are attributable to a permanent establishment or fixed place of business maintained by the non-U.S. holder in the U.S.); or (2) such non-U.S. holder is subject to backup withholding.

        In addition, a non-U.S. holder generally will not be subject to U.S. federal income or withholding tax on any gain recognized on the sale, exchange or other disposition of Aon UK shares unless: (1) such gain is effectively connected with the conduct by the non-U.S. holder of a trade or business within the U.S. (or, if a tax treaty applies, is attributable to a permanent establishment or fixed place of business maintained by the non-U.S. holder in the U.S.); (2) in the case of certain capital gains recognized by a non-U.S. holder that is an individual, such individual is present in the U.S. for 183 days or more during the taxable year in which the capital gain is recognized and certain other conditions are met; or (3) the non-U.S. holder is subject to backup withholding.

        If a non-U.S. holder is a citizen or resident of, or otherwise subject to taxation in, a country other than the U.S. or the U.K., the tax consequences of owning and disposing of Aon UK shares will depend on the applicable tax laws in such country.

64

---

Table of Contents

Material Tax Consequences to U.S. Holders if Aon UK is Ultimately Determined to Be a U.S. Corporation for U.S. Federal Tax Purposes

        Notwithstanding the foregoing, it is possible that the IRS may assert and ultimately establish that Aon UK should be treated as a U.S. corporation for U.S. federal tax purposes under Section 7874 of the Code. If the IRS were to prevail, Section 1.7874-2T(m)(1) of the Applicable Regulations provides that Aon UK should be deemed to convert to a U.S. corporation in a "reorganization" described in Section 368(a)(1)(F) of the Code on the day before the effective date of the merger. Consequently, the merger should qualify as a "reorganization" within the meaning of Section 368(a) of the Code and/or an exchange under Section 351 of the Code, and the U.S. holders of Aon Delaware shares should not be subject to U.S. federal income tax on the receipt of Aon UK shares in exchange for their Aon Delaware shares under Section 367(a) of the Code or otherwise. In such case, the adjusted tax basis of the Aon UK shares received by a U.S. holder should be equal to such holder's adjusted tax basis in its Aon Delaware shares exchanged therefor. In addition, the holding period for any Aon UK shares received by holders should include the holding period of the Aon Delaware shares exchanged therefor. The consequences of owning and disposing of the Aon UK shares should be the same as those of owning and disposing of Aon Delaware shares.

        If the IRS asserts and ultimately establishes that Aon UK should be treated as a U.S. corporation for U.S. federal tax purposes under Section 7874 of the Code, the resolution of the tax controversy may not be known until several years following the merger. Consequently, a U.S. holder that reported gain on its 2012 U.S. income tax return under Section 367(a) of Code as a result of the merger (as discussed above) may need to file an amended U.S. federal income tax return for the year in which the merger occurred in order to reflect that such U.S. holder should not have recognized gain under Section 367(a) of the Code. Furthermore, such U.S. holder may need to file an amended U.S. federal income tax return for any taxable year in which such U.S. holder disposed of any Aon UK shares received in the merger in order to reflect that the U.S. holder's adjusted tax basis in such Aon UK shares should equal such holder's adjusted tax basis in its Aon Delaware shares exchanged therefor.

        Importantly, it is possible that a tax controversy on the application of Section 7874 to the merger may not be resolved within the period of time a holder is eligible to file an amended return. As such, it is possible that certain holders may not have the opportunity to amend their 2012 U.S. income tax returns, or subsequent tax returns, as described herein. Thus, certain holders who recognize gain under Section 367(a) of the Code in 2012 could lose the opportunity to seek a refund of tax paid with respect to such gain if the IRS asserts and ultimately establishes that no gain should have been realized by such holder in 2012 because Aon UK should be treated as a U.S. corporation for U.S. federal tax purposes under Section 7874 of the Code. In addition, such holders may not be permitted to increase their adjusted tax basis in their Aon UK shares, notwithstanding that such holders recognized gain under Section 367(a) of the Code.

Information Reporting and Backup Withholding

        U.S. holders that own at least five percent (of total voting power or total value) of Aon Delaware immediately before, and/or at least five percent (of total voting power or total value) of Aon UK immediately after, the merger will be required to file with the IRS certain Section 368(a) and/or Section 351 statements. Other information reporting, including with respect to certain U.S. holders, information reporting on IRS Form 926, could also apply to the merger. Shareholders of Aon Delaware should consult their own tax advisors about the information reporting requirements that could be applicable to the exchange of Aon Delaware shares for Aon UK shares in the merger and any potential penalties associated with a failure to satisfy such requirements.

        Dividends on Aon UK shares paid within the U.S. or through certain U.S.-related financial intermediaries are subject to information reporting and may be subject to backup withholding

65

---

Table of Contents

(currently at a 28 percent rate) unless the holder (1) is a corporation or other exempt recipient (including generally non-U.S. holders who establish such foreign status) or (2) provides a taxpayer identification number and satisfies certain certification requirements. Information reporting requirements and backup withholding may also apply to the payment of proceeds from a sale of Aon UK shares within the U.S. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against the holder's U.S. federal income tax liability, provided that the holder furnishes certain required information to the IRS. Holders should consult their tax advisor regarding the application of information reporting and backup withholding to their particular situations.

        If a U.S. holder of Aon UK shares does not provide us (or our paying agent) the holder's correct taxpayer identification number or other required information, the holder may be subject to penalties imposed by the IRS.

        Section 6038D of the Code, enacted in 2010, requires an individual who holds an interest in a "specified foreign financial asset" during any taxable year beginning after March 18, 2010, to attach to such person's return of federal income tax for such taxable year, certain information with respect to such specified foreign financial asset, including the name and address of the issuer and information necessary to identify the class or issue of which such stock is a part. Temporary Regulations issued December 15, 2011 prescribe Form 8938 (Statement of Specified Financial Assets) as the appropriate means to comply with section 6038D. Stock of Aon UK would be a specified foreign financial asset unless such stock is held in an account maintained by a financial institution. Any shareholder who does not intend to hold shares of Aon UK in an account maintained by a financial institution should consult his or her own tax adviser with respect to the requirement to provide such information as an attachment to such shareholder's federal income tax return for any year in which any such shares are held in such manner.

        THE U.S. FEDERAL INCOME TAX CONSEQUENCES SUMMARIZED ABOVE ARE FOR GENERAL INFORMATION ONLY. EACH AON DELAWARE SHAREHOLDER SHOULD CONSULT HIS OR HER TAX ADVISOR AS TO THE PARTICULAR CONSEQUENCES THAT MAY APPLY TO SUCH SHAREHOLDER.

U.K. Tax Considerations

Material Tax Consequences to Shareholders

Scope of Discussion

        The following paragraphs are the opinion of our Counsel and are intended as a general guide to current U.K. tax law and HMRC practice applying as at the date of this document (both of which are subject to change at any time, possibly with retrospective effect) in respect of the taxation of capital gains, the taxation of dividends paid by Aon UK and stamp duty and SDRT on the transfer of Aon UK shares. They relate only to persons who are beneficial owners of the Aon UK shares, or the "Shareholders." These paragraphs may not relate to certain classes of holders of the Aon UK shares, such as employees or directors of Aon UK or its affiliates, persons who are connected with Aon UK, insurance companies, charities, collective investment schemes, pension schemes or persons who hold Aon UK shares otherwise than as an investment, or individuals who are not domiciled in the U.K. These paragraphs do not describe all of the circumstances in which Shareholders may benefit from an exemption or relief from taxation. It is recommended that all Shareholders obtain their own taxation advice. In particular, non-U.K. resident or domiciled Shareholders are advised to consider the potential impact of any relevant double tax agreements.

66

---

Table of Contents

Taxation of Merger

Capital Gains Tax

        At the effective time of the merger, an individual stockholder of Aon Delaware who is resident or ordinarily resident in the U.K. will be treated as disposing of his or her stock in Aon Delaware for a consideration equal to the market value of the stock at that time, which may, depending on the stockholder's individual circumstances, give rise to a chargeable gain or an allowable loss for the purposes of capital gains tax. An individual stockholder who is neither resident nor ordinarily resident in the U.K. will not be chargeable to capital gains tax on capital gains arising on the disposal of his or her stock in Aon Delaware, unless the stockholder carries on a trade, profession or vocation in the U.K. through a branch or agency in the U.K. and the stock was acquired, used in or for the purposes of the branch or agency or used in or for the purposes of the trade, profession or vocation carried on by the stockholder through the branch or agency. In these circumstances, the stockholder will be treated as disposing of their stock in Aon Delaware for a consideration equal to the market value of the stock at that time, which may, depending on the stockholder's individual circumstances, give rise to a chargeable gain or an allowable loss for the purposes of capital gains tax. The rates of capital gains tax on chargeable gains for the tax year 2011/2012 are 18 percent (for basic rate taxpayers) and 28 percent (for higher and additional rate taxpayers).

        The merger should not be treated as giving rise to a distribution which will be chargeable to income tax. The individual stockholder will be treated as acquiring the Aon UK shares for a consideration equal to his or her market value at the time of the merger for capital gains tax purposes.

Corporation Tax

        At the effective time of the merger, a corporate stockholder of Aon Delaware who is resident in the U.K. will be treated as disposing of its stock in Aon Delaware for a consideration equal to the market value of the stock at that time, which may, depending on the stockholder's individual circumstances, give rise to a chargeable gain or an allowable loss for the purposes of corporation tax. A corporate stockholder which is not resident in the U.K. will not be liable for corporation tax on chargeable gains accruing on the disposal of its stock in Aon Delaware unless it carries on a trade in the U.K. through a permanent establishment in the U.K. and the stock was acquired, used in or for the purposes of the permanent establishment or used in or for the purposes of the trade carried on by the stockholder through the permanent establishment. In these circumstances, the non-U.K. resident stockholder of the Aon Delaware stock may, depending on its individual circumstances, be chargeable to corporation tax on chargeable gains arising from a disposal of its Aon Delaware stock. The merger should not be treated as giving rise to a distribution which will be chargeable to corporation tax.

        The full rate of corporation tax on chargeable gains is 26 percent in the financial year 2011 and 25 percent in the financial year 2012, although small companies may be entitled to claim the small companies rate of tax, in which case chargeable gains will be subject to corporation tax at rates of between 20 percent and 26 or 25 percent (as appropriate). A corporate stockholder of Aon Delaware will be entitled to an indexation allowance in computing the amount of a chargeable gain accruing on the disposal of its Aon Delaware stock, which will provide relief for the effects of inflation by references to movements in the U.K. retail price index. If the conditions of the substantial shareholding exemption set out in s.192A and Schedule 7AC of the Taxation of Chargeable Gains Act 1992 are satisfied in relation to a chargeable gain accruing to a corporate holder of Aon Delaware stock, the chargeable gain will be exempt from corporation tax. The conditions of the substantial shareholding exemption which must be satisfied will depend on the individual circumstances of the corporate stockholder. One of the conditions of the substantial shareholding exemption which must be satisfied is that the stockholder must have held a substantial shareholding in Aon Delaware throughout a twelve-month period beginning not more than two years before the day on which the disposal takes place.

67

---

Table of Contents

Ordinarily, a corporate stockholder will not be regarded as holding a substantial shareholding in Aon Delaware unless it (whether alone or together with other group companies) holds not less than 10 percent of Aon Delaware's ordinary share capital. The corporate stockholder will be treated as acquiring the Aon UK shares for a consideration equal to their market value at the effective time of the merger for corporation tax purposes.

Taxation of Dividends

Withholding Tax

        Dividends paid by Aon UK will not be subject to any withholding or deduction for or on account of U.K. tax, irrespective of the residence or the individual circumstances of the Shareholders.

Income Tax

        An individual Shareholder who is resident or ordinarily resident in the U.K. may, depending on his or her individual circumstances, be subject to income tax on dividends received from Aon UK. An individual Shareholder who is not resident or ordinarily resident in the U.K. will not be chargeable to income tax on dividends received from Aon UK, unless the Shareholder carries on (whether solely or in partnership) any trade, profession or vocation through a branch or agency in the U.K. and the Aon UK shares are used by or held by or for that branch or agency. In these circumstances, the non-U.K. resident Shareholder may, depending on his or her individual circumstances, be chargeable to income tax on dividends received from Aon UK.

        The rate of income tax which is chargeable on dividends received in the tax year 2011/2012 is 32.5 percent (if taxed at the higher rate) and 42.5 percent (if taxed at the additional rate, which applies to income chargeable to income tax in excess of £150,000). An individual's dividend income is treated as the top slice of their total income which is chargeable to income tax. Individual Shareholders who are resident in the U.K. will be entitled to a tax credit equal to one-ninth of the amount of the dividend received from Aon UK, which will be taken into account in computing the gross amount of the dividend which is chargeable to income tax. The tax credit will be credited against the Shareholder's liability (if any) to income tax on the gross amount of the dividend. An individual Shareholder who is not subject to income tax on dividends received from Aon UK will not be entitled to claim payment of the tax credit in respect of such dividends. The right of an individual Shareholder who is not resident in the U.K. to a tax credit will depend on his or her individual circumstances.

Corporation Tax

        Unless an exemption is available as discussed below, a corporate Shareholder that is resident in the U.K. will be subject to corporation tax on dividends received from Aon UK. A corporate Shareholder that is not resident in the U.K. will not be subject to corporation tax on dividends received from Aon UK unless the Shareholder carries on a trade in the U.K. through a permanent establishment in the U.K. and the dividends form part of the profits of a trade carried on through or from the permanent establishment or the Aon UK shares are used by, for or held by or for, the permanent establishment. In these circumstances, the non-U.K. resident corporate Shareholder may, depending on its individual circumstances and if the exemption discussed below is not available, be chargeable to corporation tax on dividends received from Aon UK.

        The full rate of corporation tax chargeable on dividends received from Aon UK is 26 percent in financial year 2011 and 25 percent in financial year 2012, although companies with small profits may be entitled to claim the small profits rate of tax. If dividends paid by Aon UK fall within an exemption from corporation tax set out in Part 9A of the Corporation Tax Act 2009, the receipt of the dividend by a corporate Shareholder will be exempt from corporation tax. Generally, the conditions for exemption from corporation tax on dividends paid by Aon UK should be satisfied, although the conditions which

68

---

Table of Contents

must be satisfied in any particular case will depend on the individual circumstances of the corporate Shareholders.

        Shareholders that are regarded as small companies should generally be exempt from corporation tax on dividends received from Aon UK, unless the dividends are made as part of a tax advantage scheme. Corporate Shareholders that are not regarded as small companies should generally be exempt from corporation tax on dividends received from Aon UK on the basis that the Aon UK shares should be regarded as non-redeemable ordinary shares. Alternatively, corporate Shareholders that are not small companies should also generally be exempt from corporation tax on dividends received from Aon UK if they hold Aon UK shares which represent less than 10 percent of the issued share capital of Aon UK, would be entitled to less than 10 percent of the profits available for distribution to holders of the issued share capital of Aon UK and would be entitled on a winding up to less than 10 percent of the assets of Aon UK available for distribution to holders of its issued share capital. In certain limited circumstances, the exemption from corporation tax will not apply to such Shareholders if a dividend is made as part of a scheme which has a main purpose of falling within the exemption from corporation tax.

Taxation of Capital Gains

Capital Gains Tax

        A disposal of Aon UK shares by an individual Shareholder who is resident or ordinarily resident in the U.K. may, depending on his or her individual circumstances, give rise to a chargeable gain or an allowable loss for the purposes of capital gains tax. An individual Shareholder who temporarily ceases to be resident or ordinarily resident in the U.K. for a period of less than five years and who disposes of his or her Aon UK shares during that period of temporary non-residence may be liable to capital gains tax on a chargeable gain accruing on the disposal on his or her return to the U.K. under certain anti-avoidance rules. An individual Shareholder who is neither resident nor ordinarily resident in the U.K. will not be chargeable to capital gains tax on capital gains arising on the disposal of their Aon UK shares unless the Shareholder carries on a trade, profession or vocation in the U.K. through a branch or agency in the U.K. and the Aon UK shares were acquired, used in or for the purposes of the branch or agency or used in or for the purposes of the trade, profession or vocation carried on by the Shareholder through the branch or agency. In these circumstances, the non-U.K. resident Shareholder may, depending on his or her individual circumstances, be chargeable to capital gains tax on chargeable gains arising from a disposal of their Aon UK shares. The rates of capital gains tax on chargeable gains for the tax year 2011/2012 are 18 percent (for basic rate taxpayers) and 28 percent (for higher and additional rate taxpayers).

Corporation Tax

        A disposal of Aon UK shares by a corporate Shareholder which is resident in the U.K. may give rise to a chargeable gain or an allowable loss for the purposes of corporation tax. A corporate Shareholder that is not resident in the U.K. will not be liable for corporation tax on chargeable gains accruing on the disposal of its Aon UK shares unless it carries on a trade in the U.K. through a permanent establishment in the U.K. and the Aon UK shares were acquired, used in or for the purposes of the permanent establishment or used in or for the purposes of the trade carried on by the Shareholder through the permanent establishment. In these circumstances, the non-U.K. resident Shareholder may, depending on its individual circumstances, be chargeable to corporation tax on chargeable gains arising from a disposal of its Aon UK shares.

        The full rate of corporation tax on chargeable gains is 26 percent in financial year 2011 and 25 percent in financial year 2012, although companies with small profits may be entitled to claim the small profits rate of tax. Corporate Shareholders will be entitled to an indexation allowance in

69

---

Table of Contents

computing the amount of a chargeable gain accruing on a disposal of the Aon UK shares, which will provide relief for the effects of inflation by reference to movements in the U.K. retail price index. If the conditions of the substantial shareholding exemption set out in s.192A and Schedule 7AC of the Taxation of Chargeable Gains Act 1992 are satisfied in relation to a chargeable gain accruing to a corporate Shareholder, the chargeable gain will be exempt from corporation tax. The conditions of the substantial shareholding exemption which must be satisfied will depend on the individual circumstances of the corporate Shareholder. One of the conditions of the substantial shareholding exemption which must be satisfied is that the corporate Shareholder must have held a substantial shareholding in Aon UK throughout a twelve-month period beginning not more than two years before the day on which the disposal takes place. Ordinarily, a corporate Shareholder will not be regarded as holding a substantial shareholding in Aon UK unless it (whether alone, or together with other group companies) directly holds not less than 10 percent of Aon UK's ordinary share capital.

Stamp Duty and Stamp Duty Reserve Tax

        The discussion below relates to holders of Aon UK shares wherever resident (but not to holders such as market makers, brokers, dealers and intermediaries, to whom special rules apply).

Transfer of Aon UK Shares

        Any instrument of transfer of Aon UK shares that are not held by a nominee for the purposes of the provision of clearance services will attract stamp duty at a rate of 0.5 percent of the consideration for the transfer. An unconditional agreement for such transfer, or a conditional agreement which subsequently becomes unconditional, will be liable to SDRT generally at the rate of 0.5 percent of the consideration for the transfer; but such liability will be cancelled if the agreement is completed by a duty stamped instrument of transfer within six years of the date of the agreement, or if the agreement was conditional, the date the agreement became unconditional. Where stamp duty is paid, any SDRT previously paid will be repaid on the making of an appropriate claim. Stamp duty and SDRT are normally paid by the purchaser.

Transfer of Aon UK Shares Held by a Nominee As Part of a Clearance Service

        No stamp duty need, in practice, be paid on the acquisition or transfer of the beneficial ownership of Aon UK shares held by a nominee as part of a clearance service, provided that any instrument of transfer or contract for sale is not executed in the U.K. and remains at all times outside the U.K. and the transfer does not relate to any matter or thing done or to be done in the U.K. An agreement for the transfer of such Aon UK shares will not give rise to a SDRT liability.

        THE U.K. TAX CONSEQUENCES SUMMARIZED ABOVE ARE FOR GENERAL INFORMATION ONLY. EACH AON DELAWARE STOCKHOLDER SHOULD CONSULT HIS OR HER TAX ADVISOR AS TO THE PARTICULAR U.K. TAX CONSEQUENCES THAT MAY APPLY TO SUCH STOCKHOLDER.

Material U.K. Tax Considerations Relating to and After the Merger

        The following paragraphs are intended to be a general summary of certain material U.K. tax consequences relating to the merger based on current English law relating to corporation tax, stamp duty and stamp duty reserve tax and on Counsel's understanding of the current administrative policies and practices of HMRC published in writing prior to the date hereof. This summary, and all other statements, do not otherwise take into account or anticipate any changes in law or practice, whether legislative, governmental or judicial decision or action, which could possibly be made effective on a retroactive basis. This summary does not address all U.K. corporation tax, stamp duty or SDRT

70

---

Table of Contents

consequences that may be relevant to Aon Delaware, Aon UK or any of their subsidiaries, and is not intended to be, nor should it be construed to be, legal or tax advice to any person.

The Merger

        We expect that none of Aon Delaware, Aon UK or any of their subsidiaries will incur UK corporation tax as a result of completion of the merger.

        Subject to certain exemptions, a charge to stamp duty or SDRT generally arises under U.K. law on the issue, transfer or appropriation of shares of companies incorporated in the U.K. to the issuer of depositary receipts, its nominee or agent. Such stamp duty or SDRT is payable by the issuer of the depositary receipts. The rate of such stamp duty or SDRT is generally 1.5 percent of either (i) in the case of an issue of shares, the issue price of the shares concerned, (ii) in the case of a transfer of shares, the amount or value of the consideration for the transfer, or (iii) in the case of an appropriation of shares, the value of the shares concerned. However, in view of the judgment of the European Court of Justice in HSBC Holdings plc, Vidacos Nominees Ltd v HMRC Case C-569/07 given on October 1, 2009, such charge to stamp duty or SDRT may be prohibited by Council Directive 69/335/EEC of July 17, 1969 concerning indirect taxes on the raising of capital (as amended).

        The U.K. Government is considering the judgment of the Court and has not announced what action (if any) it intends to take in relation to the stamp duty and SDRT treatment of the issue, transfer or appropriation of shares of companies incorporated in the U.K. to the issuer of depositary receipts. Accordingly, with respect to the transfer of Aon UK shares to EES on December 15, 2011, SDRT at the rate of 1.5 percent on the value of such shares was paid, which was equivalent to the capital subscribed for them in issue. The SDRT has been paid on the basis that all rights to claim repayment of the SDRT are reserved. Any claim for payment of overpaid SDRT, based on the HSBC Holdings judgment or otherwise, must be submitted to HMRC within six years after the date on which the SDRT was paid.

        An exemption from SDRT applies in respect of transfers of chargeable securities between a depositary receipt system and a clearance system, save in respect of transfers from an EU system to a non-EU system where no SDRT charge arose in respect of the prior transfer into the transferor system. HMRC has confirmed to Aon Delaware's advisers that Cede & Co (as nominee for The Depository Trust Company) forms part of a clearance system, and therefore that the transfer from EES to Cede & Co planned to take place prior to the consummation of the merger will not attract SDRT.

After the Merger

General

        Subject to any available exemptions or other forms of relief, a U.K. resident company, such as Aon UK, is subject to corporation tax on its worldwide income profits and chargeable gains. The current rate of corporation tax (except for small companies and in respect of certain offshore oil and gas extraction activities) is 26 percent.

Dividend Exemption

        A U.K. resident company, such as Aon UK, is exempt from corporation tax in respect of certain broad classes of dividend or other distributions (except of a capital nature) received from companies, whether they are resident in the U.K. or overseas, including dividends or other distributions from controlled companies and in respect of non-redeemable ordinary shareholdings.

71

---

Table of Contents

Substantial Shareholding Exemption

        A U.K. resident company, such as Aon UK, which is a shareholder of a trading group is also exempt from corporation tax in respect of capital gains arising from the actual or deemed disposal of qualifying shareholdings in qualifying companies, whether they are resident in the U.K. or elsewhere. For this purpose, a capital distribution is treated as a deemed disposal of the shares of the distributing company.

Controlled Foreign Companies

        Subject to certain exemptions and other forms of relief, a U.K. resident company, such as Aon UK, is liable to corporation tax on the income profits (whether or not distributed) of any controlled company which is resident in a foreign jurisdiction and is subject to a lower level of taxation on those profits. A controlled company will be regarded as being subject to a lower level of taxation if the amount of foreign income tax on its profits is less than three-quarters of the corresponding corporation tax that would be payable in the U.K. if the company were resident in the U.K. Any such foreign income tax on the profits of the controlled company is generally creditable for U.K. corporation tax purposes.

        On December 6, 2011, the U.K. Government published draft legislation proposing major reforms to the existing CFC regime. These are expected to be included in the Finance Bill 2012, which will be laid before Parliament in the spring of 2012. The Government has proposed that the new regime would have effect for accounting periods of companies beginning on or after Royal Assent to the Finance Bill (which normally occurs in late July), but that this is subject to further consultation.

        Aon UK has received a ruling from HMRC confirming that a temporary exemption from the CFC regime in respect of foreign companies that have recently come under control of a U.K. company will apply in respect of all material subsidiaries of Aon UK following the merger. The draft legislation published on December 6, 2011 does not contain a similar temporary exemption. However, in a policy document published on January 31, 2012, the Government indicated that temporary exemptions applying under the existing regime would be grandfathered under the new rules. If enacted, this is expected to have the effect of maintaining the temporary exemption until December 31, 2014 (except to the extent that the group undertakes any steps which have the effect of invalidating it).

        No comfort can be obtained from HMRC in respect of legislation that is yet to be enacted, but it is expected that this exemption will, if passed, apply to all material subsidiaries of Aon UK for a period of up to three years following the merger. Further, based on analysis of the Government's published proposals and draft legislation, we are of the view that the proposed reforms as a whole will not have a material adverse effect on Aon UK. Subject to the final form of this legislation, Aon UK intends to seek an appropriate ruling from HMRC that any appropriate exemptions from the CFC regime will apply.

72

---

Table of Contents

DESCRIPTION OF CLASS A ORDINARY SHARES OF AON UK

General

        The following information is a summary of the material terms of the Class A Ordinary Shares, nominal (i.e. par) value $0.01 per share, as specified in the form of Aon UK's articles of association and which are proposed to be adopted prior to the consummation of the merger, which we refer to as the "New Articles." You are encouraged to read the New Articles, which are included as Annex B to this proxy statement/prospectus. See also "Comparison of Rights of Stockholders/Shareholders."

        Pursuant to the merger agreement, each issued and outstanding share of common stock of Aon Delaware will be converted into the right to receive one Class A Ordinary Share. All of the issued Class A Ordinary Shares are fully paid and not subject to any further calls or assessments by Aon UK. There are no conversion rights, redemption provisions or sinking fund provisions relating to any Class A Ordinary Shares that will be delivered in connection with the merger.

        Under English law, persons who are neither residents nor nationals of the U.K. may freely hold, vote and transfer the Aon UK shares in the same manner and under the same terms as U.K. residents or nationals.

Share Capital

        As of the date of this proxy statement/prospectus, there are 345,523,558 Class A Ordinary Shares in issue. Following the merger, Aon UK intends to cancel any excess Class A Ordinary Shares not delivered in the merger.

        As of the date of this proxy statement/prospectus there are, and immediately following the merger there will be, 500,000 Class B Ordinary Shares in issue.

        As of the date of this proxy statement/prospectus and immediately prior to the consummation of the merger, the Aon UK Board is or will be authorized to allot up to a total of an additional number of shares as follows:

        (a)   415,000,000 Class A Ordinary Shares;

        (b)   25,000,000 Preference Shares, or "Preference Shares," which Preference Shares are a class of shares of Aon UK that may be issued by the Aon UK Board pursuant to the New Articles. The Preference Shares will have such rights and nominal (i.e. par) value as the Aon UK Board shall determine at the time of allotment and issuance and may be issued in one or more classes or series with or without voting rights attached to them, with the Aon UK Board to determine the existence of such voting rights and, if any, the ranking of such voting rights in relation to the other shares in the capital of Aon UK. The Aon UK Board will also determine any other terms and conditions of the Preference Shares, including with regards to their rights: (i) to receive dividends (which may include, without limitation, the right to receive preferential or cumulative dividends); (ii) to distributions made by Aon UK on a winding up, and (iii) to be convertible into, or exchangeable for, shares of any other class or classes or of any other series of the same or any other class or classes of shares, at such prices or prices or at such rates of exchange and with such adjustments as may be determined by the Aon UK Board. Preference Shares may be issued as redeemable shares, at the option of the Aon UK Board; and

        (c)   1 Class C Ordinary Share, which it is anticipated will be used to effect a reduction of capital of Aon UK (please see "Proposal 2—Implementation of a Reduction of Capital of Aon UK" for more information). The Class C Ordinary Share is of a class of shares of Aon UK that may be issued by the Aon UK Board pursuant to the New Articles and is denominated in US Dollars with a nominal (i.e. par) value of $0.01. The Class C Ordinary Share must be issued without voting rights attached to it. The Class C Ordinary Share will rank equally with all other ordinary shares in the capital of Aon

73

---

Table of Contents

UK for any dividend declared and for any distribution made on a winding up. The Class C Ordinary Share may be issued as a redeemable share, at the option of the Aon UK Board.

        The Class A Ordinary Shares and the Class B Ordinary Shares are together referred to as the "Ordinary Shares." The outstanding Class B Ordinary Shares will remain outstanding after the merger and will have the same rights and privileges in all respects as the Class A Ordinary Shares, with the exception that the Class B Ordinary Shares will have no voting rights to the extent that they are held by one of Aon UK's subsidiaries (including Aon Delaware). If issued, the rights of holders of Preference Shares will be determined by the Aon UK Board in accordance with the New Articles.

Dividends

        Subject to the Companies Act, the Aon UK Board may declare a dividend to be paid to the shareholders in accordance with their respective rights and interests in Aon UK, and may fix the time for payment of such dividend. The Aon UK Board may from time to time declare and pay (on any class of shares of any amounts and in any currency) dividends on its issued share capital only out of its distributable reserves, defined as accumulated realized profits less accumulated realized losses, and not out of share capital, which includes share premiums. Realized reserves are determined with generally accepted accounting principles at the time the relevant accounts are prepared. Aon UK will not be permitted to make a distribution if, at the time, the amount of its net assets is less than the aggregate of its issued and paid-up share capital and undistributable reserves or to the extent that the distribution will reduce the net assets below such amount. In connection with the merger, Aon UK is seeking to ensure that sufficient distributable reserves will be available to permit dividends, distributions or share repurchases following the closing. Please see "Risk Factors—English law will require that we meet certain additional financial requirements before we declare dividends and repurchase shares following the merger," "Proposal 1—Approval of the Adoption of the Merger Agreement—Dividends," and "Proposal 2—Implementation of a Reduction of Capital of Aon UK" for more information.

        There are no fixed dates on which entitlement to dividends arise on any of the Class A Ordinary Shares. The Aon UK Board may direct the payment of all or any part of a dividend to be satisfied by distributing specific assets, in particular paid up shares or debentures of any other company. The New Articles also permit a scrip dividend scheme under which shareholders may be given the opportunity to elect to receive fully paid Class A Ordinary Shares instead of cash, with respect to all or part of future dividends. Where Class A Ordinary Shares are held by or for the benefit of an Aon UK subsidiary (including Aon Delaware), such subsidiary will have no voting rights and is expected to waive its entitlement to any dividends or distributions, including any scrip dividends, bonus shares or dividends or distributions of property or debentures of any other company.

        If a shareholder owes any money to Aon UK relating in any way to any class of Aon UK shares, the Aon UK Board may deduct any of this money from any dividend on the relevant shares, or from other money payable by Aon UK in respect of these shares. Money deducted in this way may be used to pay the amount owed to Aon UK.

        Unclaimed dividends and other amounts payable by Aon UK can be invested or otherwise used by directors for the benefit of Aon UK until they are claimed under English law. A dividend or other money remaining unclaimed for a period of twelve years after it first became due for payment will, if the Aon UK Board so resolves, be forfeited and cease to be owing to the shareholder.

Voting Rights

        At a general meeting any resolutions put to a vote must be decided on a poll.

        Subject to any rights or restrictions as to voting attached to any class of shares and subject to disenfranchisement (i) in the event of non-payment of any call or other sum due and payable in respect

74

---

Table of Contents

of any shares not fully paid, (ii) in the event of any non-compliance with any statutory notice requiring disclosure of an interest in shares, (iii) with respect to any shares held by any subsidiary of Aon UK or (iv) in the event of any non-compliance with the information requirements set out in the New Articles in respect of a resolution that a qualifying member (or members) is (or are) proposing to bring before a meeting, every shareholder (other than Aon Delaware or any other subsidiary of Aon UK) who (being an individual) is present in person or (being a corporation) is present by a duly authorized corporate representative at a general meeting of Aon UK will have one vote for every share of which he or she is the holder, and every person present who has been appointed as a proxy shall have one vote for every share in respect of which he or she is the proxy.

        In the case of joint holders, the vote of the person whose name stands first in the register of shareholders and who tenders a vote, whether in person or by proxy, is accepted to the exclusion of any votes tendered by any other joint holders.

        The necessary quorum for a general shareholder meeting is the shareholders who together represent at least the majority of the voting rights of all the shareholders entitled to vote at the meeting, present in person or by proxy (i.e., any shares whose voting rights have been disenfranchised pursuant to the Companies Act shall be disregarded for the purposes of determining a quorum).

        An annual general meeting shall be called by not less than 21 clear days' notice and no more than 60 days' notice. For all other general meetings except general meetings properly requisitioned by shareholders, such meetings shall be called by not less than 14 clear days' notice and no more than 60 days' notice. The notice of meeting may also specify a time by which a person must be entered on the register in order to have the right to attend or vote at the meeting. The number of shares then registered in their respective names shall determine the number of votes a person is entitled to cast at that meeting.

        An appointment of proxy (whether in hard copy form or electronic form) must be received by Aon UK before the time for holding the meeting or adjourned meeting at which the person named in the appointment of proxy proposes to vote and at such time as may be specified by the Board in compliance with the provisions of the Companies Act. An appointment of proxy not received or delivered in accordance with the New Articles is invalid under English law.

        To the extent that as a result of the merger any Class A Ordinary Shares are held by or for the benefit of any of Aon UK's subsidiaries, including Aon Delaware, then under the Companies Act such Class A Ordinary Shares will not have voting rights.

Winding Up

        In the event of a voluntary winding-up of Aon UK, the liquidator may, on obtaining any sanction required by law, divide among the shareholders the whole or any part of the assets of Aon UK, whether or not the assets consist of property of one kind or of different kinds.

        The liquidator may also, with the same authority, transfer the whole or any part of the assets to trustees upon any trusts for the benefit of the shareholders as the liquidator decides. No past or present shareholder can be compelled to accept any asset which could subject him or her to a liability.

Preemptive Rights and New Issues of Shares

        Under Section 549 of the Companies Act, directors are, with certain exceptions, unable to allot securities without being authorized either by the shareholders in a general meeting or by the New Articles pursuant to Section 551 of the Companies Act. In addition, under the Companies Act, the issuance of equity securities that are to be paid for wholly in cash (except shares held under an employees' share scheme) must be offered first to the existing equity shareholders in proportion to the respective nominal (i.e., par) values of their holdings on the same or more favorable terms, unless a

75

---

Table of Contents

special resolution (i.e. 75 percent of votes cast) to the contrary has been passed in a general meeting of shareholders or the articles of association otherwise provide an exclusion from this requirement (which exclusion can be for a maximum of five years after which shareholders approval would be required to renew the exclusion). In this context, equity securities generally means in relation to Aon UK, Ordinary Shares (being shares other than shares which with respect to dividends or capital, carry a right to participate only up to a specified amount in a distribution) and all rights to subscribe for or convert securities into such shares.

        A provision in the New Articles will authorize the directors (generally and unconditionally), for a period up to five years from the date on which the New Articles are adopted by Aon UK to allot equity securities, or to grant rights to subscribe for or to convert or exchange any security into shares of Aon UK, up to 415,000,000 Class A Ordinary Shares, up to 25,000,000 Preference Shares and up to one Class C Ordinary Share, and exclude preemptive rights in respect of such issuances for the same period of time. Such authority will continue for five years and thereafter it must be renewed, but we may seek renewal for additional five year terms more frequently. Aon UK may, before the expiration of any such authority, make an offer or agreement which would or might require Aon UK shares to be allotted (or rights to be granted) after such expiration, and the directors may allot shares or grant rights in pursuance of such an offer or agreement as if the authority to allot had not expired.

        Subject to the provisions of the Companies Act and to any rights attached to any existing shares, any Aon UK shares may be issued with, or have attached to them, such rights or restrictions as the shareholders of Aon UK may by ordinary resolution determine, or, where the appropriate authorizations are in place in the New Articles, the Aon UK Board may determine such rights or restrictions.

        The Companies Act prohibits an English company from issuing shares for no consideration, including with respect to grants of restricted stock made pursuant to equity incentive plans. Accordingly, the nominal value of the shares issued upon the lapse of restrictions or the vesting of any restricted stock award or any other share-based grant underlying any Class A Ordinary Shares must be paid pursuant to the Companies Act.

Disclosure of Interests in Shares

        Section 793 of the Companies Act gives Aon UK the power to require persons whom it knows has, or whom it has reasonable cause to believe has, or within the previous three years has had, any ownership interest in any shares (which we refer to as the "default shares") to disclose prescribed particulars of those shares. For this purpose "default shares" includes any shares allotted or issued after the date of the Section 793 notice in respect of those shares. Failure to provide the information requested within the prescribed period after the date of sending the notice will result in sanctions being imposed against the holder of the "default shares" as provided within the Companies Act.

        Under the New Articles, Aon UK will also withdraw certain voting rights of "default shares" if the relevant holder of "default shares" has failed to provide the information requested within the prescribed period after the date of sending the notice, depending on the level of the relevant shareholding (and unless the Aon UK Board decides otherwise).

Alteration of Share Capital/Repurchase of Shares

        Aon UK may from time to time by ordinary resolution of its shareholders:

•

increase its share capital by allotting new shares in accordance with the authority contained in the relevant shareholder resolution and the New Articles;



•

consolidate and divide all or any of its share capital into shares of a larger nominal amount than the existing shares; and

76

---

Table of Contents

•

subdivide any of its shares into shares of a smaller nominal amount than its existing shares.

        Subject to the Companies Act and to any rights the holders of any Aon UK shares may have, Aon UK may purchase any of its own shares of any class (including any redeemable shares, if the Aon UK Board should decide to issue any) by way of "off market purchases" with the prior approval of 75 percent of shareholders by special resolution. Such approval may be for a specific purchase or constitute a general authority lasting for up to five years from the date of the special resolution, and renewal of such approval for additional five year terms may be sought more frequently. A special resolution, which would authorize the repurchase per year of up to all of the share capital outstanding as of the beginning of each fiscal year, is anticipated to be adopted prior to the effective time of the merger. However, shares may only be repurchased out of distributable reserves or, subject to certain exceptions, the proceeds of a fresh issue of shares made for that purpose.

Transfer of Shares

        Aon UK's New Articles allow shareholders to transfer all or any of their shares by instrument of transfer in writing in any usual form or in any other form which is permitted by the Companies Act and is approved by the Aon UK Board. The instrument of transfer must be executed by or on behalf of the transferor and (in the case of a transfer of a share which is not fully paid) by or on behalf of the transferee.

        The Aon UK Board may refuse to register a transfer:

        (1)   if the shares in question are not fully paid;

        (2)   if it is with respect to more than one class of shares;

        (3)   if it is with respect to shares on which Aon UK has a lien;

        (4)   if it is in favor of more than four persons jointly;

        (5)   if it is not duly stamped (if such a stamp is required);

        (6)   if it is not presented for registration together with the share certificate and evidence of title as the Aon UK Board reasonably requires; or

        (7)   in certain circumstances, if the holder has failed to provide the required particulars to Aon UK as described under "—Disclosure of Interests in Shares" above.

        If the Aon UK Board refuses to register a transfer of a share, it shall, within two months after the date on which the transfer was lodged with Aon UK, send to the transferee notice of the refusal, together with its reasons for refusal.

General Meetings and Notices

        The notice of a general meeting shall be given to the shareholders (other than any who, under the provisions of the New Articles or the terms of allotment or issue of shares, are not entitled to receive notice), to the Aon UK Board and to the auditors. Under English law, Aon UK is required to hold an annual general meeting of shareholders within 6 months from the day following the end of its fiscal year and, subject to the foregoing, the meeting may be held at a time and place determined by the Aon UK Board whether within or outside of the U.K..

Liability of Aon UK and its Directors and Officers

        The New Articles provide that English courts have exclusive jurisdiction with respect to any suits brought by shareholders (in their capacity as such) against Aon UK or its directors. See "Comparison

77

---

Table of Contents

of Rights of Stockholders/Shareholders—Liability of Directors and Officers" for a discussion of the inability of an English company to exempt directors and officers from certain liabilities.

Anti-takeover Provisions

        The level of anti-takeover provisions with respect to Aon UK differs from that with respect to Aon Delaware by virtue of the differences between the DGCL and the Companies Act and the differences between the certificate of incorporation and by-laws of Aon Delaware and the New Articles. The provisions summarized below do not include those provisions required by the Companies Act. The provisions of the New Articles summarized below may have the effect of discouraging, delaying or preventing hostile takeovers, including those that might result in a premium being paid over the market price of Class A Ordinary Shares, as applicable, and discouraging, delaying or preventing changes in control or management of Aon UK.

        An English public limited company is potentially subject to the U.K. City Code on Takeovers and Mergers, or "Takeover Code" if, among other factors, its central place of management and control are within the U.K., the Channel Islands or the Isle of Man. The Takeover Panel will generally look to the residency of a company's directors to determine where it is centrally managed and controlled. The Takeover Panel has confirmed that, based upon Aon UK's current and intended plans for its directors and management, the Takeover Code will not apply to Aon UK. It is possible that, in the future, circumstances could change that may cause the Takeover Code to apply to Aon UK.

Issuance of Preference Shares

        The Aon UK Board will have the authority, without further action of its shareholders for a period of five years (from the date of adoption of the New Articles), but subject to its statutory and fiduciary duties, and to the requirements of English law on Aon UK, to issue up to 25,000,000 Preference Shares, in one or more series and to fix the powers, preferences, rights and qualifications, limitations or restrictions thereof. Such authority will continue for five years (from the date of adoption of the New Articles) and thereafter it must be renewed, but we may seek renewal for additional five year terms more frequently. The issuance of Preference Shares on various terms could adversely affect the holders of Class A Ordinary Shares. The potential issuance of Preference Shares may discourage bids for Class A Ordinary Shares at a premium over the market price, may adversely affect the market price of Class A Ordinary Shares and may discourage, delay or prevent a change of control of Aon UK.

78

---

Table of Contents

COMPARISON OF RIGHTS OF STOCKHOLDERS/SHAREHOLDERS

        Your rights as a stockholder of Aon Delaware are governed by the DGCL and Aon Delaware's certificate of incorporation and by-laws. After the merger, you, through your ownership of Class A Ordinary Shares, will have the rights of a shareholder of Aon UK, and your rights will generally be governed by English law and the New Articles. Immediately following the completion of the merger, each Class A Ordinary Share issued in connection with the merger will be held through the facilities of DTC. As a result, your rights will also be governed by the rules and procedures of DTC and the relationship between you and the bank, broker or other financial institution who holds Class A Ordinary Shares on your behalf.

        The economic and voting rights of Aon Delaware common stock and Class A Ordinary Shares are similar. However, there are differences between your rights under the DGCL and under applicable English law. In addition, there are differences between Aon Delaware's certificate of incorporation and by-laws and the New Articles.

        The following discussion is a summary of the material differences in your rights that would result from the merger and assuming you hold Class A Ordinary Shares. As such, this summary does not cover all the differences between applicable English law and the DGCL affecting corporations and their stockholders or all of the differences between Aon Delaware's certificate of incorporation and by-laws and the New Articles. While we believe this summary is accurate in all material respects, the following descriptions are qualified in their entirety by reference to the complete text of the relevant provisions of applicable English law, the DGCL, Aon Delaware's certificate of incorporation and by-laws and the New Articles. We encourage you to read those laws and documents. A copy of the New Articles is attached to this proxy statement/prospectus as Annex B. For information as to how you can obtain a copy of Aon Delaware's certificate of incorporation and by-laws, see "Where You Can Find More Information."

79

---

Table of Contents

80

---

Table of Contents

81

---

Table of Contents

82

---

Table of Contents

83

---

Table of Contents

84

---

Table of Contents

85

---

Table of Contents

86

---

Table of Contents

87

---

Table of Contents

88

---

Table of Contents

89

---

Table of Contents

90

---

Table of Contents

91

---

Table of Contents

92

---

Table of Contents

93

---

Table of Contents

94

---

Table of Contents

95

---

Table of Contents

96

---

Table of Contents

97

---

Table of Contents

98

---

Table of Contents

99

---

Table of Contents

100

---

Table of Contents

101

---

Table of Contents

102

---

Table of Contents

103

---

Table of Contents

104

---

Table of Contents

PROPOSAL 2—IMPLEMENTATION OF A REDUCTION OF CAPITAL OF AON UK

        Under English law, Aon UK will only be able to declare dividends, make distributions or repurchase shares out of "distributable reserves". "Distributable reserves" are a company's accumulated, realized profits, so far as not previously utilized by distribution or capitalization, less its accumulated, realized losses, so far as not previously written off in a reduction or reorganization of capital duly made. Immediately after the merger, as a newly formed public limited company, Aon UK will not have distributable reserves. In order to have sufficient distributable reserves to continue paying quarterly dividends and to repurchase shares on the anticipated schedule for the foreseeable future Aon UK will seek to have an amount, approximately equal to the sum of Aon Delaware's retained earnings and additional paid-in capital less Aon Delaware's outstanding indebtedness as of a date on or around closing created as distributable reserves following a reduction of capital of Aon UK implemented through a customary and conventional one-time court proceeding. Please see "Risk Factors—English law will require that we meet certain additional financial requirements before we declare dividends and repurchase shares following the merger" and "Proposal 1—Approval of the Adoption of the Merger Agreement—Dividends" for more information.

        The current shareholder of Aon UK (which is Aon Delaware) has passed a resolution to reduce the capital of Aon UK to allow the creation of distributable reserves following the merger. Based on the retained earnings, additional paid-in capital and outstanding indebtedness of Aon Delaware as of September 30, 2011, distributable reserves of Aon UK resulting from this resolution would be approximately $7.8 billion. This Proposal 2 is seeking the approval of Aon Delaware's current stockholders, as the future stockholders of Aon UK, for that action. In connection with this process, a revaluation reserve will arise in Aon UK in respect of its indirect holding in Aon Delaware. Aon UK will capitalize the revaluation reserve by issuing a share in Aon UK (the Class C Ordinary Share) to an Aon group company, which would allow Aon UK to undertake a reduction of capital in respect of such share.

        If this Proposal 2 to reduce Aon UK's capital is approved by Aon Delaware's stockholders and the merger is completed, we will seek to obtain the approval of the English Companies Court through a customary process, which is required for the creation of distributable reserves to be effective, as soon as practicable following the transaction. The court has discretion as to whether to approve the reduction of capital including whether the interests of creditors are adequately safeguarded. The approval of the English Companies Court is expected to be received within four weeks after the completion of the merger. In the future, earnings of Aon UK will form part of the distributable reserves of Aon UK.

        The approval of this Proposal 2 is not a condition to the completion of the merger. If stockholders do not approve the proposal to approve an implementation of a reduction of capital of Aon UK, Aon UK would undertake other efforts to allow it to declare dividends, make distributions or repurchase shares following the completion of the merger. These efforts may include certain customary intra-group reorganizations, which are established and often-used alternatives for the creation of distributable reserves in a U.K. public limited company, but which we believe to be less advantageous than the proposed court-approved reduction in capital.

Recommendation and Required Vote

        The affirmative vote of a majority of the shares of common stock of Aon Delaware present in person or represented by proxy at the special meeting and entitled to vote with respect to Proposal 2 is required to approve Proposal 2. ACCORDINGLY, OUR BOARD RECOMMENDS THAT STOCKHOLDERS VOTE "FOR" THE PROPOSAL TO IMPLEMENT A REDUCTION OF CAPITAL OF AON UK THROUGH A CUSTOMARY COURT-APPROVED PROCESS SO THAT IT WILL HAVE SUFFICIENT DISTRIBUTABLE RESERVES TO DECLARE DIVIDENDS, MAKE DISTRIBUTIONS OR REPURCHASE SHARES FOLLOWING THE CLOSING.

105

---

Table of Contents

PROPOSAL 3—ADVISORY VOTE ON CERTAIN COMPENSATION PAYABLE IN CONNECTION WITH THE TRANSACTION

        The Dodd Frank Wall Street Reform and Consumer Protection Act of 2010, or the "Dodd-Frank Act", and Rule 14a-21(c) under the Exchange Act, require that Aon Delaware provides its stockholders with the opportunity to vote to approve, on a non-binding, advisory basis, the compensation that might be received by named executive officers of a company in connection with certain mergers and other corporate transactions, which the Dodd-Frank Act and Rule 14a-21 refer to as "golden parachute" compensation. The applicable rules and regulations interpreting these provisions are very broad, and may contemplate an advisory vote on any type of compensation, whether present, deferred or contingent, that is based on or otherwise relates to, among other things, a merger. None of Aon Delaware's named executive officers are entering into any new severance arrangements or other traditional "golden parachute" arrangements in connection with the merger and, in fact, each of Aon Delaware's named executive officers that is relocating to the U.K. has agreed to waive any entitlement to severance that he or she would have had as a result of being required to relocate to the U.K.

        However, because the compensation of Aon Delaware's named executive officers will be changed in connection with their relocation to the U.K., and because the reorganization of Aon Delaware's operations is being accomplished through a merger, Aon Delaware is requesting its stockholders' approval, on a non-binding, advisory basis, of the compensation that might be received by certain of Aon Delaware's named executive officers in connection with the merger, as described under "Proposal 1—Approval of the Adoption of the Merger Agreement—Interests of Directors and Executive Officers in the Merger—Executive Officers of Aon Delaware", including the associated narrative discussion, and therefore are asking stockholders to adopt the following resolution:

"RESOLVED, that the compensation that may be paid to certain of Aon Delaware's named executive officers in connection with the merger, as described under "Proposal 1—Approval of the Adoption of the Merger Agreement—Interests of Directors and Executive Officers in the Merger—Executive Officers of Aon Delaware", including the associated narrative discussion, and the agreements or understandings pursuant to which such compensation might be paid or become payable, are hereby APPROVED."

        The relocation benefits were approved by the Organization and Compensation Committee of the Board after consulting with its independent compensation consultant. The benefits provided are customary for expatriate assignments for the Company and other employers in the industry and are intended to keep the relocated executive "whole" on a total rewards basis, be transparent and equitable and reflect best practices and benchmarks of industry counterparts.

        The vote on this Proposal 3 is a vote separate and apart from each of the other proposals. Accordingly, you may vote to approve this Proposal 3 and vote not to approve any or all of the other proposals. Because the vote is advisory in nature only, it will not be binding on Aon Delaware regardless of whether the merger is approved. Approval of the non-binding, advisory proposal with respect to the compensation that might be received by Aon Delaware's named executive officers in connection with the merger is not a condition to completion of the merger, and failure to approve this advisory matter will have no effect on the vote to approve the merger. In addition, because Aon Delaware is contractually obligated to pay such compensation to these individuals, regardless of the outcome of this advisory vote, such compensation will be payable, subject only to the conditions applicable thereto, if the merger is approved.

Recommendation and Required Vote

        The affirmative vote of a majority of the shares of common stock of Aon Delaware present in person or represented by proxy at the special meeting and entitled to vote with respect to Proposal 3 is required to approve Proposal 3. ACCORDINGLY, OUR BOARD RECOMMENDS THAT STOCKHOLDERS VOTE "FOR" THE NON-BINDING, ADVISORY PROPOSAL REGARDING COMPENSATION THAT MIGHT BE RECEIVED BY AON DELAWARE'S NAMED EXECUTIVE OFFICERS IN CONNECTION WITH THE MERGER.

106

---

Table of Contents

PROPOSAL 4—ADJOURNMENT PROPOSAL

        Aon Delaware is asking its stockholders to consider and vote upon a proposal to approve one or more adjournments of the special meeting, if necessary or appropriate, to permit further solicitation of proxies in favor of Proposal 1.

        If the number of shares of common stock present in person or represented by proxy at the special meeting voting in favor of Proposal 1 to adopt the merger agreement and approve the merger and the reorganization is insufficient to approve Proposal 1 at the time of the special meeting, then Aon Delaware may move to adjourn the special meeting in order to enable its Board to solicit additional proxies in respect of such proposal. In that event, Aon Delaware stockholders will be asked to vote only upon the adjournment proposal, and not on any other proposal, including Proposal 1.

        In this proposal, you are being asked to authorize the holder of any proxy solicited by Aon Delaware's Board to vote in favor of granting discretionary authority to the proxy or attorney-in-fact to adjourn the special meeting one or more times for the purpose of soliciting additional proxies. If Aon Delaware's stockholders approve the adjournment proposal, Aon Delaware could adjourn the special meeting and any adjourned session of the special meeting and use the additional time to solicit additional proxies, including the solicitation of proxies from stockholders that have previously returned properly executed proxies or authorized a proxy by using the Internet or telephone. Among other things, approval of the adjournment proposal could mean that, even if Aon Delaware has received proxies representing a sufficient number of votes against the approval of Proposal 1 such that the proposal would be defeated, Aon Delaware could adjourn the special meeting without a vote on Proposal 1 and seek to obtain sufficient votes in favor of approval of Proposal 1 to obtain approval of that proposal.

Recommendation and Required Vote

        The affirmative vote of a majority of the shares of common stock of Aon Delaware present in person or represented by proxy at the special meeting and entitled to vote with respect to Proposal 4 is required to approve Proposal 4. OUR BOARD RECOMMENDS THAT STOCKHOLDERS VOTE "FOR" THE APPROVAL OF THIS PROPOSAL.

107

---

Table of Contents

LEGAL MATTERS

        The validity of the Class A Ordinary Shares of Aon UK registered hereby has been passed upon for Aon UK by Freshfields Bruckhaus Deringer LLP.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

        The consolidated financial statements of Aon Delaware appearing in Aon Delaware's Annual Report on Form 10-K for the year ended December 31, 2010 and the effectiveness of Aon Delaware's internal control over financial reporting as of December 31, 2010 have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

        Aon Delaware is subject to the informational requirements of the Exchange Act and in accordance therewith files annual, quarterly and current reports, proxy statements and other information with the SEC. You may read and copy any reports, statements or other information we file at the SEC's public reference room at 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the public reference rooms. Aon Delaware's SEC filings also are available to the public from commercial document retrieval services and at the SEC's website at www.sec.gov.

        Aon UK has filed a Registration Statement on Form S-4 with the SEC to register its Class A Ordinary Shares in connection with the merger. This proxy statement/prospectus is a part of that registration statement and constitutes a prospectus of Aon UK under applicable U.S. securities laws in addition to being the proxy statement of Aon Delaware for the special meeting. This proxy statement/prospectus is not intended to be and is not a prospectus for purposes of the U.K. Financial Services Authority's Prospectus Rules.

        In addition to the information set forth in this proxy statement/prospectus, SEC rules allow Aon Delaware and Aon UK to "incorporate by reference" information into this proxy statement/prospectus, which means that Aon Delaware can disclose important information to you by referring you to another document filed separately with the SEC. The information incorporated by reference is deemed to be part of this proxy statement/prospectus, except for any information therein that is superseded by information set forth in this proxy statement/prospectus. We incorporate by reference the documents listed below and any additional documents that we will file with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act (excluding any information "furnished" but not "filed") following the date of this document, but prior to the date of the special meeting. These documents contain important information about Aon Delaware. The documents incorporated by reference are:

1.

Annual Report on Form 10-K for the year ended December 31, 2010, filed with the SEC on February 25, 2011;



2.

Proxy Statement on Schedule 14A, filed with the SEC on April 8, 2011;



3.

Quarterly Report on Form 10-Q for the quarter ended March 31, 2011, filed with the SEC on May 5, 2011;



4.

Quarterly Report on Form 10-Q for the quarter ended June 30, 2011, filed with the SEC on August 4, 2011;



5.

Quarterly Report on Form 10-Q for the quarter ended September 30, 2011, filed with the SEC on November 4, 2011; and

108

---

Table of Contents

6.

Current Reports on Form 8-K (in each case, other than information and exhibits "furnished" to and not "filed" with the SEC in accordance with SEC rules and regulations) filed with the SEC on March 1, 2011, March 8, 2011, March 23, 2011, May 16, 2011, May 24, 2011, May 25, 2011, May 27, 2011, June 15, 2011, October 24, 2011, December 20, 2011, January 13, 2012, January 23, 2012 and January 26, 2012.

        If you are a stockholder of record or beneficial owner, we may already have sent you some of the documents incorporated by reference, but you can obtain any of them from us or the SEC. Stockholders of record, beneficial owners, and any other person to whom a proxy statement is delivered, may obtain without charge a copy of documents that we incorporate by reference into this proxy statement/prospectus (including exhibits, but only if specifically requested) by requesting them by telephone at (312) 381-1000 or in writing at the following address:

Aon Corporation
200 East Randolph Street
Chicago, IL 60601
Attention: Investor Relations

        In addition, you may obtain copies of some of this information by accessing Aon's website at www.aon.com under the heading "Investor Relations," and then under the link "SEC Filings." If you would like to request documents from us, please do so no later than five business days before the special meeting, to assure that you will receive them before the special meeting.

        You should rely only on the information contained or incorporated by reference into this proxy statement/prospectus to consider and vote upon the adoption of the merger agreement. We have not authorized anyone to provide you with information that is different from what is contained in this proxy statement/prospectus. The date of this proxy statement/prospectus can be found on the first page. You should not assume that the information contained in this proxy statement/prospectus is accurate as of any date other than that date, and neither the mailing of this proxy statement/prospectus to stockholders nor the issuance of Class A Ordinary Shares in the merger shall create any implication to the contrary.

OTHER INFORMATION

Information Concerning Stockholder Proposals Made at Special Meeting

        Aon Delaware's by-laws limit the business that may be transacted at a special meeting of stockholders to matters brought before the meeting pursuant to the notice of the meeting. Accordingly, Aon Delaware's stockholders may not submit other proposals for consideration at the special meeting.

Stockholder Proposals for Next Annual Meeting

        Aon Delaware held its 2011 annual meeting of stockholders on May 20, 2011. The deadline for submitting a stockholder proposal for inclusion in Aon Delaware's proxy statement and form of proxy pursuant to Rule 14a-8 under the Exchange Act for its 2012 annual meeting of stockholders has passed. However, if the merger is not completed, stockholder proposals that are intended to be presented at Aon Delaware's 2012 annual meeting, but that are not intended to be considered for inclusion in Aon Delaware's proxy statement and proxy related to that meeting, or nominations of a candidate for election as a director, must have been received no earlier than January 21, 2012 and no later than February 20, 2012. Any nominations or proposals must provide the information required by Aon Delaware's by-laws and comply with any applicable laws and regulations. All submissions should be made to the Office of the Secretary at Aon Delaware's principal offices at 200 East Randolph Street, Chicago, Illinois 60601.

109

---

Table of Contents

Shareholder Proposals After Merger

        Upon the completion of the merger, Aon UK will become the successor issuer of Aon Delaware for purposes of U.S. securities laws. The deadline for submitting a stockholder proposal for inclusion in Aon UK's proxy statement and form of proxy pursuant to Rule 14a-8 under the Exchange Act for its 2012 annual meeting of stockholders has passed. If you desire to bring a matter before a meeting of the shareholders of Aon UK and the proposal is submitted outside the process of Rule 14a-8, a shareholder must follow the procedures set out under the Companies Act and the New Articles as set out in Annex B.

Postponements of the Special Meeting

        At any time prior to convening the special meeting, Aon Delaware may postpone the special meeting one or more times for any reason without the approval of Aon Delaware's stockholders. If the special meeting were postponed, Aon Delaware could use the additional time to solicit additional proxies, including the solicitation of proxies from stockholders that have previously returned properly executed proxies or authorized a proxy by using the Internet or telephone.

        Any adjournment or postponement of the special meeting for the purpose of soliciting additional proxies will allow Aon Delaware stockholders who have already sent in their proxies to revoke them at any time prior to their use at the special meeting.

Householding of Proxy Materials

        We participate, and some brokers, banks, trusts and other nominee record holders may be participating, in the practice of "householding" proxy materials. This procedure allows multiple stockholders residing at the same address the convenience of receiving a single proxy statement/prospectus and any other proxy materials. You may request a separate copy of the proxy statement/prospectus by calling (866) 647-8869. You also may request paper copies when prompted after you vote at Georgeson, 199 Water Street, 26^th Floor, New York, New York 10038.

110

---

Table of Contents

ANNEX A

AGREEMENT AND PLAN OF MERGER AND REORGANIZATION

        This AGREEMENT AND PLAN OF MERGER AND REORGANIZATION (this "Agreement") is entered into as of January 12, 2012, by and among Aon Corporation, a Delaware corporation ("Aon Delaware"), and Market Mergeco Inc., a Delaware corporation and a wholly-owned indirect subsidiary of Aon Delaware ("Mergeco").

RECITALS:

        A.    The Board of Directors of Aon Delaware has determined that it is advisable and in the best interests of Aon Delaware's stockholders to reorganize so that Mergeco will merge with and into Aon Delaware with Aon Delaware as the surviving corporation (the "Merger").

        B.    The Board of Directors of Aon Delaware has approved the Merger, this Agreement and, to the extent applicable to Aon Delaware, the other transactions described herein or contemplated hereby, pursuant to which Aon Delaware will be the surviving corporation in the Merger and will become a wholly-owned subsidiary of Aon Holdings LLC, a newly formed Delaware limited liability company ("Aon Intermediate"), and whereby each issued share of common stock, par value US$1.00 per share, of Aon Delaware ("Aon Delaware Common Stock"), other than those shares of Aon Delaware Common Stock held by Aon Delaware in treasury or by any subsidiaries of Aon Delaware, shall be converted into the right to receive one Class A Ordinary Share, nominal value US$0.01 per share (collectively, the "Class A Ordinary Shares") of Aon Global Limited, a private limited company incorporated under English law, which, prior to the Effective Time (defined below), will re-register as a public limited company to be named "Aon plc" (or a similar name) ("Aon UK"), all upon the terms and subject to the conditions set forth in this Agreement.

        C.    The Board of Directors of Mergeco has unanimously determined that it is advisable and in the best interests of Mergeco's sole stockholder to enter into the Merger and has unanimously approved the Merger, this Agreement and, to the extent applicable, the other transactions described herein or contemplated hereby.

        D.    The Merger requires, among other things, the adoption of this Agreement by the sole stockholder of Mergeco and the affirmative vote of the holders of at least two-thirds of all of the issued and outstanding shares of Aon Delaware Common Stock entitled to vote at a meeting of stockholders.

        E.    The sole stockholder of Mergeco is, immediately following the execution and delivery of this Agreement, expected to approve the Merger, this Agreement and, to the extent applicable, the other transactions described herein or contemplated hereby.

        F.     The parties intend that, for U.S. federal income tax purposes, the Merger qualifies as (i) a "reorganization" within the meaning of Section 368(a)(1) of the Internal Revenue Code of 1986, as amended (the "Code"), and (ii) an exchange under Section 351 of the Code.

        G.    The parties further intend for this Agreement to constitute a plan of reorganization within the meaning of Treasury Regulation Section 1.368-2(g).

A-1

---

Table of Contents

AGREEMENT:

        NOW THEREFORE, in consideration of the foregoing and of the covenants and agreements contained herein, and of other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto hereby agree as follows:

ARTICLE I
THE MERGER

        Section 1.1    The Merger.    Subject to the terms and conditions of this Agreement, and in accordance with the Delaware General Corporation Law (the "DGCL"), at the Effective Time, Mergeco shall merge with and into Aon Delaware in accordance with this Agreement, and the separate corporate existence of Mergeco shall thereupon cease. From and after the Effective Time and in accordance with the DGCL, Aon Delaware shall continue as the surviving corporation in the Merger (sometimes hereinafter referred to as the "Surviving Corporation"), becoming a wholly-owned subsidiary of Aon Intermediate.

        Section 1.2    Filing Certificate of Merger; Effective Time.    As soon as practicable following the satisfaction or, to the extent permitted by applicable law, waiver of the conditions set forth in Article V, if this Agreement shall not have been terminated as provided in Section 6.1, the parties shall cause a certificate of merger (the "Certificate of Merger") meeting the requirements of Section 251(c) of the DGCL to be properly executed and filed in accordance with such section and otherwise make all other filings or recordings as required by the DGCL in connection with the Merger. The Merger shall become effective upon the filing of the Certificate of Merger (or such later date and time as the parties hereto shall agree, which shall be reflected in the Certificate of Merger) (the "Effective Time").

        Section 1.3    Effects.    The Merger shall have the effects set forth in this Agreement and Section 259 of the DGCL.

ARTICLE II
CHARTER DOCUMENTS;
DIRECTORS AND OFFICERS OF
SURVIVING CORPORATION

        Section 2.1    Name of the Surviving Corporation.    The name of the Surviving Corporation shall be "Aon Corporation."

        Section 2.2    Certificate of Incorporation of the Surviving Corporation.    From and after the Effective Time, the Certificate of Incorporation of Mergeco in effect immediately prior to the Effective Time shall be the Certificate of Incorporation of the Surviving Corporation, until duly amended in accordance with its terms and applicable law.

        Section 2.3    Bylaws of Surviving Corporation.    From and after the Effective Time, the Bylaws of Mergeco in effect immediately prior to the Effective Time shall be the Bylaws of the Surviving Corporation, until duly amended in accordance with their terms and applicable law.

        Section 2.4    Directors of Surviving Corporation.    From and after the Effective Time, the directors of Mergeco immediately prior to the Effective Time shall be the directors of the Surviving Corporation, each such director to serve in such capacity until his or her earlier death, resignation or removal or until his or her successor is duly elected or appointed.

        Section 2.5    Officers of Surviving Corporation.    From and after the Effective Time, the officers of Aon Delaware immediately prior to the Effective Time shall be the officers of the Surviving Corporation, each such officer to serve in such capacity until his or her earlier death, resignation or removal or until his or her successor is duly elected or appointed.

A-2

---

Table of Contents

ARTICLE III
CANCELLATION AND CONVERSION OF STOCK

        Section 3.1    Effect on Capital Stock.    At the Effective Time, by virtue of the Merger and without any action on the part of the holder of any Aon Delaware Common Stock, any Mergeco common stock, any Class A Ordinary Shares of Aon UK, any Class B Ordinary Shares of Aon UK or any Class C Ordinary Shares of Aon UK:

        a.    Cancellation of Aon Delaware Common Stock.    Each issued share of Aon Delaware Common Stock that is owned by Aon Delaware (as a treasury share or otherwise) or by any subsidiaries of Aon Delaware (excluding any shares of Aon Delaware Common Stock (x) held by Aon Delaware or its subsidiaries in connection with any market making activities or proprietary trading activities and (y) shares held in trust, managed, custodial or nominee accounts and the like, including in respect of any employee benefit plans of Aon Delaware or its subsidiaries, or held by mutual funds for which a subsidiary of Aon Delaware acts as investment advisor or in a similar capacity) immediately prior to the Effective Time shall automatically be cancelled and retired and shall cease to exist, and no consideration shall be delivered or deliverable hereunder in connection with such cancellation.

        b.    Conversion of Remaining Aon Delaware Shares.    Subject to Section 3.2(l), each issued and outstanding share of Aon Delaware Common Stock (other than the issued shares of Aon Delaware Common Stock referenced in Section 3.1(a)) shall be converted into the right to receive one (the "Exchange Ratio"), validly issued, fully paid and nonassessable Class A Ordinary Share.

        c.    Conversion of Mergeco Common Stock.    All issued and outstanding shares of common stock of Mergeco shall be converted into one validly issued, fully paid and nonassessable share of common stock, par value US$0.01 per share, of the Surviving Corporation.

        d.    Equity-Based Compensation Plans.    With effect from the Effective Time, Aon Delaware shall assign to Aon UK, and shall cause Aon UK to assume and adopt, Aon Delaware's rights and obligations under those equity compensation plans, and the underlying award agreements outstanding thereunder, that provide for the delivery of equity securities of Aon Delaware to any employee, director or other service provider as compensation for services and are not exempt from the shareholder approval requirement under New York Stock Exchange ("NYSE") listing standards and are listed on Exhibit A (collectively, the "Equity Plans"). To the extent an Equity Plan provides for awards of incentive stock options pursuant to Section 422 of the Code, approval of such plan by Aon Delaware, as the sole direct or indirect shareholder of Aon UK, shall be deemed, as of the Effective Time, to constitute shareholder approval of such Equity Plan and the issuance or delivery of Class A Ordinary Shares by Aon UK under the share limitations set forth thereunder for purposes of Section 422(b) of the Code.

        e.    Change in Control Arrangements.    With effect from the Effective Time, Aon Delaware shall assign to Aon UK, and shall cause Aon UK to assume and adopt, Aon Delaware's rights and obligations under each change-in-control agreement with a key employee and under the Aon Corporation Executive Special Severance Plan, as amended from time to time, and as listed on Exhibit A (collectively, the "CIC Arrangements").

        f.    Other Employee Benefit Plans and Employment Arrangements.    With the exception of the Equity Plans and the CIC Arrangements, Aon Delaware shall remain the plan sponsor of all other employee benefit plans and programs and employment contracts and arrangements it currently sponsors or is contractually bound by in accordance with Article IV of this Agreement (collectively, the "Unassumed Plans").

A-3

---

Table of Contents

        g.    Class B Ordinary Shares of Aon UK.    Each issued and outstanding Class B Ordinary Share, par value £0.10 per share, of Aon UK that is owned by Aon Delaware immediately prior to the Effective Time shall remain outstanding.

        h.    Class C Ordinary Shares of Aon UK.    Any issued and outstanding Class C Ordinary Share, of Aon UK that is owned by Aon Delaware immediately prior to the Effective Time shall remain outstanding.

        Section 3.2    Surrender and Exchange of Shares.    

        a.     Following the date of this Agreement and in any event prior to the Effective Time, Aon Delaware shall select a bank or trust company to act as exchange agent in connection with the Merger (together with any other bank or trust company also selected, the "Exchange Agent") for the purpose of delivering or causing to be delivered to each holder of Aon Delaware Common Stock those Class A Ordinary Shares and any cash in lieu of fractional interests to which such holder shall become entitled to receive with respect to such holder's shares of Aon Delaware Common Stock pursuant to this Article III. The Exchange Agent shall act as agent for each holder of shares of Aon Delaware Common Stock in connection therewith.

        b.     Prior to the Effective Time, Mergeco, or, after the Effective Time, the Surviving Corporation, shall deposit or cause to be deposited with the Exchange Agent, from time to time, (i) that number of Class A Ordinary Shares, in such denominations as the Exchange Agent shall specify, as are deliverable pursuant to Section 3.1, and which, unless the Company shall otherwise determine, shall be deposited with the Exchange Agent through the facilities of The Depository Trust Company ("DTC"), and (ii) the amount of cash that is payable pursuant to this Article III, in each case in respect of shares of Aon Delaware Common Stock for which Certificates are expected to be properly delivered to the Exchange Agent.

        c.     Except as Aon Delaware shall otherwise determine, all Class A Ordinary Shares and cash in lieu of fractional interests, if any, deliverable in respect of shares of Aon Delaware Common Stock held of record, other than such shares held of record by Cede & Co., as nominee for DTC, shall be not be delivered to such holders of record directly, but shall be delivered instead to a bank or trust company selected by Aon Delaware that is a participant in the facilities of DTC (the "Custodian"), which Custodian shall initially hold such shares for the benefit of such holders.

        d.     Promptly after the Effective Time, the Surviving Corporation shall cause to be mailed to each record holder, immediately prior to the Effective Time, of shares of Aon Delaware Common Stock that are represented by certificates ("Certificates"), a form of letter of transmittal, which shall specify that delivery shall be effected, and risk of loss and title to the Certificates held by such holder representing such shares of Aon Delaware Common Stock shall pass, only upon actual and proper delivery of the Certificates to the Exchange Agent.

        e.     Each holder of shares of Aon Delaware Common Stock that are represented by Certificates shall be entitled to receive in exchange for such holder's shares of Aon Delaware Common Stock that are represented by Certificates, upon surrender to the Exchange Agent of a Certificate, together with a letter of transmittal, duly completed and validly executed in accordance with the instructions thereto, and such other documents as may be required pursuant to such instructions, the number of whole Class A Ordinary Shares into which such holder's shares of Aon Delaware Common Stock represented by such holder's properly surrendered Certificates were converted in accordance with Section 3.1 and any cash dividends or other distributions that such holder has the right to receive pursuant to Section 3.2(i) and Section 3.2(l).

        f.      If delivery of Class A Ordinary Shares in respect of shares of Aon Delaware Common Stock represented by a Certificate is to be made to a person other than the person in whose name the surrendered Certificate is registered, it shall be a condition of delivery that the Certificate so

A-4

---

Table of Contents

surrendered shall be properly endorsed or shall be otherwise in proper form for transfer and that the person requesting such payment or delivery shall have paid to the Exchange Agent any transfer and other taxes required by reason of the delivery of the Class A Ordinary Shares to a person other than the registered holder of the Certificate surrendered or shall have established to the satisfaction of the Exchange Agent that such tax either has been paid or is not applicable. Until so surrendered, each Certificate shall, after the Effective Time, represent for all purposes only the right to receive upon such surrender the applicable Class A Ordinary Shares as contemplated by this Article III.

        g.     At the Effective Time, the stock transfer books of Aon Delaware shall be closed and thereafter there shall be no further registration of transfers of shares of Aon Delaware Common Stock that were outstanding prior to the Effective Time. After the Effective Time, Certificates presented to the Surviving Corporation for transfer shall be canceled and exchanged for the consideration provided for, and in accordance with the procedures set forth, in this Article III.

        h.     Any Class A Ordinary Shares to be delivered plus any cash dividend or other distribution that a former holder of shares of Aon Delaware Common Stock has the right to receive pursuant to this Article III that remains unclaimed by any former holder of shares of Aon Delaware Common Stock after the Effective Time shall be held by the Exchange Agent (or a successor agent appointed by the Surviving Corporation). Any portion of such securities or funds that remains undistributed to the former holders of shares of Aon Delaware Common Stock at the twelve month anniversary of the Effective Time shall be delivered to the Surviving Corporation, upon demand, or to its designee, and any former holder of shares of Aon Delaware Common Stock who has not theretofore complied with this Article III shall thereafter look only to the Surviving Corporation for payment of any consideration due to it hereunder. None of the Surviving Corporation, Aon UK or the Exchange Agent shall be liable to any former holder of shares of Aon Delaware Common Stock for any securities properly delivered or any amount properly paid by the Exchange Agent or its nominee, as the case may be, to a public official pursuant to applicable abandoned property, escheat or similar law nine months after the Effective Time. If any Certificate has not been surrendered prior to two years after the Effective Time (or immediately prior to an earlier date on which the Class A Ordinary Shares in respect of the Certificate would otherwise escheat to or become the property of any governmental entity) any cash, share dividends and distributions otherwise payable in respect of the Certificate shall, to the extent permitted by applicable law, become the property of the Surviving Corporation, free and clear of all claims or interest of any person previously entitled thereto.

        i.      No dividends or other distributions with respect to Class A Ordinary Shares deliverable with respect to the shares of Aon Delaware Common Stock shall be paid to the holder of any unsurrendered Certificates until after those Certificates are surrendered as provided in this Article III. After surrender, there shall be delivered and/or paid to the holder of the Class A Ordinary Shares delivered in exchange therefor, without interest, (A) at the time of surrender, the dividends or other distributions payable with respect to those Class A Ordinary Shares with a record date on or after the date of the Effective Time and a payment date on or prior to the date of this surrender and not previously paid and (B) at the appropriate payment date, the dividends or other distributions payable with respect to those Class A Ordinary Shares with a record date on or after the date of the Effective Time but with a payment date subsequent to surrender.

        j.      No holder of Aon Delaware Common Stock will be entitled to exercise voting rights with respect to Class A Ordinary Shares deliverable with respect to such Aon Delaware Common Stock until after such holder has surrendered the Certificates, if any, representing such Aon Delaware Common Stock as provided in this Article III.

A-5

---

Table of Contents

        k.     In the event that any Certificate shall have been lost, stolen or destroyed, upon the holder's compliance with the replacement requirements established by the Exchange Agent, including, if necessary, the posting by the holder of a bond in customary amount as indemnity against any claim that may be made against it with respect to the Certificate, the Exchange Agent shall deliver in exchange for the lost, stolen or destroyed Certificate the applicable Class A Ordinary Shares deliverable in respect of the shares of Aon Delaware Common Stock represented by the Certificate pursuant to this Article III.

        l.      Each holder of shares of Aon Delaware Common Stock otherwise entitled to receive a fractional interest in a Class A Ordinary Share pursuant to the terms of this Article III, shall be entitled to receive, in accordance with the provisions of this Section 3.2(l), a cash payment (without interest) in lieu of that fractional interest in a Class A Ordinary Share determined by multiplying the fractional interest to which such holder would otherwise be entitled by the closing price for a Class A Ordinary Share as reported on the NYSE on the last trading day prior to the date on which the Effective Time occurs. For purposes of this Section 3.2(l), all fractional interests to which a single holder would be entitled shall be aggregated and calculations shall be rounded to three decimal places. Any cash payment in lieu of a fractional interest shall be made in U.S. dollars and shall be rounded to the nearest cent.

        m.    Notwithstanding anything in this Agreement to the contrary, the Surviving Corporation and the Exchange Agent shall be entitled to deduct and withhold from the consideration otherwise payable to any former holder of shares of Aon Delaware Common Stock pursuant to this Agreement any amounts as may be required to be deducted and withheld with respect to the making of this payment under the U.S. Internal Revenue Code of 1986, as amended, or under any provision of state, local or foreign tax law. To the extent that amounts are so withheld and paid over to the appropriate taxing authority, the Surviving Corporation shall be treated as though it withheld an appropriate amount of the type of consideration otherwise payable pursuant to this Agreement to any former holder of shares of Aon Delaware Common Stock, sold this consideration for an amount of cash equal to the fair market value of the consideration at the time of the deemed sale and paid these cash proceeds to the appropriate taxing authority.

        Section 3.3    No Dissenters' Rights.    There are no dissenters' rights or appraisal rights available to holders of Aon Delaware Common Stock under the DGCL in connection with the Merger.

ARTICLE IV
EQUITY PLANS, UNASSUMED PLANS, CIC ARRANGEMENTS AND OTHER EMPLOYMENT AGREEMENTS AND ARRANGEMENTS

        Section 4.1    Assignment and Assumption of Equity Plans.    Aon Delaware shall assign to Aon UK, and shall cause Aon UK to adopt and assume, the rights and obligations of Aon Delaware under the Equity Plans at the Effective Time.

        Section 4.2    Application of Equity Plans to Class A Ordinary Shares.    To the extent any Equity Plan provides for the grant, issuance, delivery or purchase of, or otherwise relates to, options, awards of restricted stock or other rights to purchase or receive shares of Aon Delaware Common Stock (or the right to receive benefits or amounts by reference to those shares), from and after the Effective Time, Aon Delaware or Aon UK, as applicable, shall cause such Equity Plan to be amended to provide for the grant, issuance, delivery or purchase of, or otherwise relate to, Class A Ordinary Shares, and all options or awards issued, or benefits available or based upon the value of a specified number of shares of Aon Delaware Common Stock, under such Equity Plans after the Effective Time shall entitle the holder thereof to purchase or receive, or receive payment based on, as applicable, an equal number of Class A Ordinary Shares in accordance with the terms of such Equity Plan. Except as provided in this Section 4.2 and for certain amendments to Equity Plans intended to clarify the meaning of certain

A-6

---

Table of Contents

provisions in such Equity Plans, the outstanding options, restricted stock awards or other equity-based awards or benefits available under the terms of any Equity Plan at and following the Effective Time shall, to the extent permitted by law and otherwise reasonably practicable, be subject to the same terms and conditions as under such Equity Plan and the agreements relating thereto immediately prior to the Effective Time (including, for greater certainty, having the same option exercise or measurement price). Notwithstanding the foregoing, the number of Class A Ordinary Shares, issuable or available upon the exercise, payment, issuance or availability of such option, award or benefit immediately after the Effective Time, and the option exercise or measurement price of each such option, award or benefit, shall be subject to adjustment by Aon Delaware or Aon UK, as applicable, only to the extent necessary to comply with applicable law. The foregoing adjustments shall be made in accordance with applicable law (and administrative practice of applicable governmental authorities), including but not limited to Section 409A of the Code and the U.S. Treasury Regulations promulgated thereunder, to the extent such provisions are applicable to an option, award, or benefit. Other than as set forth above, the Merger shall constitute a "reorganization" for purposes of the Equity Plans, to the extent such term is applicable, and will not affect the underlying terms or conditions of any outstanding equity awards, which shall remain subject to their original terms and conditions.

        Section 4.3    Issuance of Class A Ordinary Shares under Equity Plans.    To the extent any Class A Ordinary Shares are held by Aon Delaware after the Effective Time, Aon Delaware agrees to use such Class A Ordinary Shares for future issuances in connection with the Equity Plans, as directed by Aon UK and as permitted by applicable law.

        Section 4.4    CIC Arrangements.    Aon Delaware shall, or Aon Delaware shall cause Aon UK to, as applicable, amend such CIC Arrangements as necessary to: provide for the appropriate substitution of Aon UK in place of Aon Delaware; provide for a change-in-control definition generally consistent with the definition currently set forth in the CIC Arrangements but modified as necessary to comport with the governance and ownership structure of Aon UK; provide that the merger will not constitute a change in control under the terms of any such CIC Arrangement; comply with applicable English or U.S. corporate or tax law requirements; and make any other conforming or clarifying changes as may be necessary.

        Section 4.5    Unassumed Plans and Other Employment Agreements and Arrangements.    Aon Delaware shall amend the Unassumed Plans and employment agreements and arrangements other than the CIC Arrangements, as necessary, to: provide for the appropriate substitution of Aon UK in place of Aon Delaware, where applicable; provide that the merger will not constitute a change in control under the terms of any such plan, agreement or arrangement to the extent permitted or applicable; to comply with applicable English or U.S. corporate or tax law requirements; and to make any other conforming or clarifying changes necessary.

ARTICLE V
CONDITIONS PRECEDENT

        Section 5.1    Conditions Precedent to the Obligations of Each Party.    The obligations of the respective parties hereto to effect the Merger are subject to the satisfaction or waiver of the following conditions:

        a.     Holders of at least two-thirds of all of the issued and outstanding shares of Aon Delaware Common Stock entitled to vote at a meeting of stockholders at the record date for such actions as set by the Board of Directors of Aon Delaware shall have approved the adoption of this Agreement.

        b.     Following the approval referenced in Section 5.1(f), Aon Delaware shall have contributed all of its interests in Class A Ordinary Shares to Aon Intermediate, which shall, in turn, have contributed all such interests in Class A Ordinary Shares to Mergeco.

A-7

---

Table of Contents

        c.     Aon UK shall have re-registered as a public limited company.

        d.     Following the approval referenced in Section 5.1(a) and the contributions referred to at Section 5.1(b), Aon Delaware shall have contributed all shares of Aon Intermediate to Aon UK.

        e.     None of the parties hereto shall be subject to any decree, order or injunction of any court of competent jurisdiction, whether in the U.S., the U.K., the European Union or any other country, that prohibits the consummation of the Merger.

        f.      The registration statement on Form S-4 filed with the Securities and Exchange Commission filed by Aon UK in connection with the offer of the Class A Ordinary Shares to be delivered as consideration pursuant to the Merger shall have become effective under the U.S. Securities Act of 1933, as amended (the "Securities Act"), and no stop order with respect thereto shall be in effect.

        g.     Aon Delaware shall have received an opinion of counsel as to certain tax matters in form and acceptable to Aon Delaware.

        h.     The Class A Ordinary Shares shall have been authorized for listing on the NYSE, subject to official notice of delivery and satisfaction of other standard conditions.

        i.      Other than the filing of the Certificate of Merger provided for under Article I, all material consents and authorizations of, filings or registrations with, and notices to, any governmental or regulatory authority required of Aon Delaware, Mergeco or any of their respective subsidiaries to consummate the Merger and the other transactions contemplated hereby, including without limitation any filings required under (i) applicable U.S. state securities and "Blue Sky" laws, (ii) U.K. financial services laws, and (iii) laws governing insurance brokerage throughout the world, shall have been obtained or made.

        j.      A special resolution shall have been passed to alter the rights of the holders of the Class A Ordinary Shares to have the same rights as holders of ordinary shares of Class B Ordinary Shares, conditional upon the Merger becoming unconditional.

        k.     The Class A Ordinary Shares shall have been deemed eligible for deposit, book-entry and clearance services by DTC and its affiliates.

ARTICLE VI
TERMINATION, AMENDMENT AND WAIVER

        Section 6.1    Termination.    This Agreement may be terminated and the Merger abandoned by action of the Board of Directors of Aon Delaware at any time prior to or after approval by the stockholders of Aon Delaware of this Agreement but before the Effective Time.

        Section 6.2    Effect of Termination.    In the event this Agreement is terminated as provided in Section 6.1, this Agreement shall forthwith become void and have no effect, without any liability or obligation on the part of any party hereto.

        Section 6.3    Amendment.    This Agreement may be amended by the parties hereto at any time before or after the stockholders of Aon Delaware approve the adoption this Agreement; provided, however, that after any such adoption by the Aon Delaware stockholders, this Agreement shall not be further amended without the approval of the Aon Delaware stockholders unless any such amendment shall not require the approval of such stockholders under applicable law or under the NYSE listing rules. This Agreement may not be amended except by an instrument in writing signed on behalf of each of the parties hereto.

        Section 6.4    Waiver.    At any time prior to the Effective Time, the parties may waive compliance with any of the agreements or covenants contained in this Agreement, or may waive any of the

A-8

---

Table of Contents

conditions to consummation of the Merger contained in this Agreement. Any agreement on the part of a party to any such waiver shall be valid only if set forth in an instrument in writing signed on behalf of such party. The failure of any party to this Agreement to assert any of its rights under this Agreement or otherwise shall not constitute a waiver of such rights.

ARTICLE VII
COVENANTS

        Section 7.1    Rule 16b-3 Approval.    The parties shall take, and cause their respective subsidiaries to take, all such steps as may reasonably be required to cause the transactions contemplated by Section 3.1 and any other dispositions of Aon Delaware equity securities (including derivative securities) or acquisitions of Class A Ordinary Shares (including derivative securities thereof) in connection with this Agreement by each individual who (a) is a director or officer of Aon Delaware, or (b) at the Effective Time, is or will become a director or officer of Aon UK, to be exempt under Rule 16b-3 promulgated under the Securities Exchange Act of 1934, as amended.

ARTICLE VIII
GENERAL PROVISIONS

        Section 8.1    Assignment; Binding Effect; Benefit.    Neither this Agreement nor any of the rights, interests or obligations hereunder shall be assigned by either of the parties hereto (whether by operation of law or otherwise) without the prior written consent of the other party. Subject to the preceding sentence, this Agreement shall be binding upon and shall inure to the benefit of the parties hereto and their respective successors and assigns. Notwithstanding anything contained in this Agreement to the contrary, nothing in this Agreement, expressed or implied, is intended to confer on any person other than the parties hereto or their respective successors and assigns any rights, remedies, obligations or liabilities under or by reason of this Agreement.

        Section 8.2    Entire Agreement.    This Agreement and any documents delivered by the parties in connection herewith constitute the entire agreement between the parties with respect to the subject matter hereof and supersede all prior agreements and understandings between the parties with respect thereto.

        Section 8.3    Governing Law.    This Agreement shall be governed by and construed in accordance with the laws of the State of Delaware without regard to its rules of conflict of laws that would apply any other law.

        Section 8.4    Counterparts.    This Agreement may be executed by the parties hereto in separate counterparts, each of which when so executed and delivered shall be an original, but all such counterparts shall together constitute one and the same instrument. Each counterpart may consist of a number of copies hereof, each signed by less than all, but together signed by both of the parties hereto.

        Section 8.5    Headings.    Headings of the Articles and Sections of this Agreement are for the convenience of the parties only and shall be given no substantive or interpretative effect whatsoever.

        Section 8.6    Severability.    If any provision of this Agreement is determined by any court or arbitrator of competent jurisdiction to be invalid, illegal or unenforceable in any respect, such provision will be enforced to the maximum extent possible given the intent of the parties hereto. If such clause or provision cannot be so enforced, such provision shall be stricken from this Agreement and the remainder of this Agreement shall be enforced as if such invalid, illegal or unenforceable clause or provision had (to the extent not enforceable) never been contained in this Agreement.

[REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

A-9

---

Table of Contents

        IN WITNESS WHEREOF, the parties have executed this Agreement as of the date first written above.

   

[Signature Page to Agreement and Plan of Merger and Reorganization]

A-10

---

Table of Contents

Exhibit A

Equity Plans

        Aon Stock Incentive Plan, as amended and restated effective as of January 1, 2006, and as amended from time to time thereafter

        Amended and Restated Global Stock and Incentive Plan of Hewitt Associates, Inc.

        Aon Corporation 2011 Incentive Plan

CIC Arrangements

        Aon Corporation Executive Special Severance Plan, effective January 5, 2005, and as amended from time to time

        Severance (or Change in Control) Agreements between Aon Delaware and the following individuals:

Gregory C. Case
Christa Davies
Stephen P. McGill
Michael J. O'Connor
Baljit Dail
Gregory J. Besio
Peter Lieb
Kristi Savacool
Philip B. Clement
Matthew Levin

---

Table of Contents

Annex B

COMPANY NUMBER: 7876075

COMPANIES ACT 2006

---

A PUBLIC COMPANY LIMITED BY SHARES

---

ARTICLES OF ASSOCIATION

of

[Aon] plc

---

Table of Contents

CONTENTS

---

Table of Contents

---

Table of Contents

B-1

---

Table of Contents

B-2

---

Table of Contents

B-3

---

Table of Contents

B-4

---

Table of Contents

B-5

---

Table of Contents

B-6

---

Table of Contents

B-7

---

Table of Contents

B-8

---

Table of Contents

B-9

---

Table of Contents

B-10

---

Table of Contents

B-11

---

Table of Contents

B-12

---

Table of Contents

B-13

---

Table of Contents

B-14

---

Table of Contents

B-15

---

Table of Contents

B-16

---

Table of Contents

B-17

---

Table of Contents

B-18

---

Table of Contents

B-19

---

Table of Contents

B-20

---

Table of Contents

B-21

---

Table of Contents

B-22

---

Table of Contents

B-23

---

Table of Contents

B-24

---

Table of Contents

B-25

---

Table of Contents

B-26

---

Table of Contents

B-27

---

Table of Contents

B-28

---

Table of Contents

B-29

---

Table of Contents

B-30

---

Table of Contents

B-31

---

Table of Contents

B-32

---

Table of Contents

B-33

---

Table of Contents

B-34

---

Table of Contents

B-35

---

Table of Contents

B-36

---

Table of Contents

B-37

---

Table of Contents

B-38

---

Table of Contents

B-39

---

Table of Contents

B-40

---

Table of Contents

B-41

---

Table of Contents

B-42

---