OFFER TO PURCHASE FOR CASH
                           ALL SHARES OF COMMON STOCK
                                       OF
                      UNITED ASSET MANAGEMENT CORPORATION
                                       AT
                              $25.00 NET PER SHARE
     (SUBJECT TO ADJUSTMENT AS PROVIDED IN THE ENCLOSED OFFER TO PURCHASE)
                                       BY
                              OM ACQUISITION CORP.
                           A WHOLLY-OWNED SUBSIDIARY
                                       OF
                                 OLD MUTUAL PLC

     THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00
                          MIDNIGHT,
  NEW YORK CITY TIME, ON FRIDAY, AUGUST 11, 2000, UNLESS THE
                      OFFER IS EXTENDED.

                                                                   July 17, 2000

To Our Clients:

    Enclosed for your consideration are the Offer to Purchase, dated July 17,
2000 (the 'OFFER TO PURCHASE'), and the related Letter of Transmittal (which,
together with any amendments and supplements thereto, collectively constitute
the 'OFFER') in connection with the offer by OM Acquisition Corp., a Delaware
corporation ('PURCHASER') and a wholly-owned subsidiary of Old Mutual plc, a
public limited company incorporated in England and Wales, to purchase all
outstanding shares of common stock, par value $0.01 per share (the 'SHARES'), of
United Asset Management Corporation, a Delaware corporation (the 'COMPANY'), at
a purchase price of $25.00 per Share, subject to downward adjustment as provided
in the Offer to Purchase, net to you in cash, without interest thereon (the
'SHARE PRICE'). WE ARE THE HOLDER OF RECORD OF SHARES HELD FOR YOUR ACCOUNT. A
TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND
PURSUANT TO YOUR INSTRUCTIONS. THE ENCLOSED LETTER OF TRANSMITTAL IS FURNISHED
TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER SHARES HELD
BY US FOR YOUR ACCOUNT.

    We request instructions as to whether you wish us to tender any or all of
the Shares held by us for your account, upon the terms and subject to the
conditions contained in the Offer.

    Your attention is invited to the following:

        1. The offer price is $25.00 per Share, subject to downward adjustment
    as provided in the Offer to Purchase, net to you in cash, without interest.

        2. The Offer is being made for all outstanding Shares.

        3. The Board of Directors of the Company has unanimously determined that
    the Offer and the Merger (as defined in the Offer to Purchase) are fair to,
    advisable and in the best interests of the Company and its stockholders, has
    approved the Offer and the Merger Agreement (as defined in the Offer to
    Purchase) and the transactions contemplated thereby and recommends that the
    Company's stockholders accept the Offer and tender all of their Shares
    pursuant to the Offer.

        4. The Offer and withdrawal rights expire at 12:00 midnight, New York
    City time, on Friday, August 11, 2000, unless the Offer is extended.

        5. The Offer is conditioned upon, among other things, there being
    validly tendered and not properly withdrawn prior to the Expiration Date (as
    defined in the Offer to Purchase) that number of Shares which consitutes at
    least a majority of the Company's common stock then issued and










    outstanding on a fully-diluted basis. The Offer is also subject to certain
    other conditions contained in the Offer to Purchase. See the Introduction
    and Sections 1, 14 and 15 of the Offer to Purchase.

        6. Any stock transfer taxes applicable to the transfer and sale of
    Shares to Purchaser pursuant to the Offer will be paid by Purchaser, except
    as otherwise provided in Instruction 6 of the Letter of Transmittal.

    Except as disclosed in the Offer to Purchase, Purchaser is not aware of any
state in which the making of the Offer is prohibited by administrative or
judicial action pursuant to any valid state statute. In any jurisdiction in
which the securities, blue sky or other laws require the Offer to be made by a
licensed broker or dealer, the Offer will be deemed to be made on behalf of
Purchaser by the Dealer Managers or one or more registered brokers or dealers
licensed under the laws of such jurisdiction.

    If you wish to have us tender any or all of your Shares, please so instruct
us by completing, executing and returning to us the instruction form attached to
this letter. An envelope to return your instructions to us is enclosed. If you
authorize the tender of your Shares, all such Shares will be tendered unless
otherwise specified on the attachment to this letter. Your instructions should
be forwarded to us in ample time to permit us to submit a tender on your behalf
prior to the expiration of the Offer.

    Notwithstanding any other provision hereof, payment for Shares accepted for
payment pursuant to the Offer will in all cases be made only after timely
receipt by the Depositary of Share certificates for, or of Book-Entry
Confirmation (as defined in the Offer to Purchase) with respect to, such Shares,
a properly completed and duly executed Letter of Transmittal (or facsimile
thereof), together with any required signature guarantees (or, in the case of a
book-entry transfer, an Agent's Message (as defined in the Offer to Purchase)),
and any other documents required by the Letter of Transmittal. Accordingly,
payment might not be made to all tendering stockholders at the same time, and
will depend upon when Share certificates or Book-Entry Confirmations of such
Shares are received into the Depositary's account at the Book-Entry Transfer
Facility (as defined in the Offer to Purchase).

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                        INSTRUCTIONS WITH RESPECT TO THE
                           OFFER TO PURCHASE FOR CASH
                           ALL SHARES OF COMMON STOCK
                                       OF
                      UNITED ASSET MANAGEMENT CORPORATION
                                       AT
                          $25.00 NET PER SHARE IN CASH
          (SUBJECT TO ADJUSTMENT AS PROVIDED IN THE OFFER TO PURCHASE)

    The undersigned acknowledge(s) receipt of your letter and the enclosed Offer
to Purchase dated July 17, 2000 and the related Letter of Transmittal in
connection with the Offer by OM Acquisition Corp., a Delaware corporation and a
wholly-owned subsidiary of Old Mutual plc, a public limited company incorporated
in England and Wales, to purchase all outstanding shares of common stock, par
value $0.01 per share (the 'Shares'), of United Asset Management Corporation, a
Delaware corporation, at a price of $25.00 per Share, subject to downward
adjustment as provided in the Offer to Purchase, net to the seller in cash,
without interest thereon, upon the terms and subject to the conditions in the
Offer to Purchase and related Letter of Transmittal.

    This will instruct you to tender the number of Shares indicated below (or if
no number is indicated below, all Shares) held by you for the account of the
undersigned, upon the terms and subject to the conditions contained in the
Offer.

Number of Shares to Be Tendered*

 .............................. Shares

Dated:  ....................... , 2000

                                           .....................................

                                           .....................................
                                                       SIGNATURE(S)

                                           .....................................

                                           .....................................
                                                      PRINT NAME(S)

                                           .....................................

                                           .....................................
                                                       ADDRESS(ES)

                                           .....................................
                                              AREA CODE AND TELEPHONE NUMBER

                                           .....................................
                                             TAX ID OR SOCIAL SECURITY NUMBER
- ---------
* Unless otherwise indicated, it will be assumed that all Shares held by us for
  your account are to be tendered.

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