Officially one of biggest concerns was with the cloud gaming market monopoly as they could have all of Activision titles on Game Pass so Microsoft conceded that thier Activision cloud game rights would be given to Ubisoft.
A. The U.S. relented. The UK was never a massive player in this deal, they were just one of the first to loudly dissent. While the U.S. was blocking the deal, they were able to easily point to the FTC and sit on their hands. They caved pretty soon after.
B. Microsoft made some promises. Ubisoft is going to get game streaming rights, which means if Microsoft wants to make any of their games available on Game Pass, Ubisoft has to agree and Microsoft will have to pay Ubi for the privilege.
Microsoft was willing to leave the UK market as its not big enough compared to the value the aquisition would be valued.
The UK argued for Microsofts cloud gaming dominamce. To appease thr CMA, microsoft gave ubisoft all streaming rights to act/blizz games for like a ridiculously long timr.