They can block the companies from trading in that country ultimately if the merger happens. Could Microsoft Activision in theory pull out of the entire UK mark

They can block the companies from trading in that country ultimately if the merger happens. Could Microsoft Activision in theory pull out of the entire UK market and still merge (not just gaming, stop selling Office in the UK for example)? Yes. Will they? Absolutely not. There are significant chunks of Microsoft shareholders who don't want to do this deal in the first place and they certainly aren't going to torch the profitable bits of Microsoft for the gaming division, which has terrible financial performance.