Their sales there are in the neighborhood of $5B annually (~2% of their overall run rate). Add a guess of $500m for Activision's UK sales for a total of $5.5B.
The hit to sales would be temporary; the UK government would eventually capitulate as their citizens revolt at not being able to buy Windows or Office. (Yes, there are other options but a sudden loss of access to Microsoft products would be devastating.)
Based on the numbers, it's not clear to me that the UK has the leverage to stop this merger. If they were still part of the EU, this calculus wouldn't begin to make sense.