I'm not worried about the deal. And worse case scenario, even if UK blocks it, Microsoft will go to court and chances are that it will be overturned as CMA has had a lot of blocks overturned Past that, Microsoft isn't huge in UK for Xbox anyway and they could simply pull out of the region and that's it.
I can easily see small AA publishers like Paradox or Sega. I don't think there would be any issues with either if they were to be acquired by Microsoft. Only companies I see as a potential issue is Take Two Interactive because of GTA and to a lesser extent, Electronic Arts. Everyone else, I don't think anyone would even care.