Ubisoft can’t assign the agreement with MS to a third party (someone buying them, for example) unless MS agrees (in some specific cases). In those cases, if MS delays, conditions or withhelds the consent, the CMA will be able to intervene (pages 4-5 from the remedies acceptance):
In addition, the Proposed Undertakings enable the CMA to be involved in any arbitration proceedings arising in relation to assignment of the Ubisoft Divestment Agreement to a third party. Under the terms of the Ubisoft Divestment Agreement, assignment of the Ubisoft Divestment Agreement by Ubisoft to a third party requires the Parties' consent in certain circumstances, which cannot be unreasonably withheld, delayed, or conditioned. In the event of any dispute in relation to the granting of the Parties' consent being referred to arbitration, the Proposed Undertakings require the Parties to promptly notify the CMA of the dispute and provide the CMA with the opportunity to provide representations on the proposed assignment.