It doesn't have to be like this, though. If this deal is blocked, but Activision still wants to sell to Microsoft, wouldn't it only require Activision to remove itself from UK territories to avoid regulation? Activision would have to do this before acquisition, meaning it isn't Microsoft circumventing regulation, but Activision.If MS turns around to the UK (which is somewhere between the 5th and 7th largest economy in the world depending on the day of the week) and says "stuff you, we're not willing to be regulated. If we can't do what we want we're taking our ball home", then everybody else will take notice. The EU, all EU members, etc. and if they have any sense the US too. That's a lot of GDP, and prime government pork to be putting at risk for some video games.
Regardless, I don't think Microsoft would necessarily have to fully pull out of the UK to have an impact on UK politics or regulation. They could simply stop investing in the UK, and perhaps move some of their UK studios to neighboring countries. We've just seen Disney change course on invenstments in Florida because of a feud with that state's administration, costing the state construction jobs and future taxes. I won't pretend to be a Brexit expert, but aren't parts of or all of Ireland somehow both parts of the UK and EU? If the EU has approved the deal, and MS and ABK move to Ireland, would they have both regulatory approval and guaranteed access to the UK market from the Belfast agreement?
Also, it isn't just other governments that are watching, it's other industry as well. We've already head statement coming from Microsoft claiming that the UK is closed for business. It isn't hard to think that other companies might think twice before setting up shop there, if they think they can get less regulatory scrutiny elsewhere.