Microsoft and Activision both have assets in the UK, both sell products and services in the UK, and so the UK has a right to regulate their activity in the UK market.
Similar situation to other jurisdictions (EU, China, India, etc.)
Similar situation to other jurisdictions (EU, China, India, etc.)
Microsoft and Activision do both have various subsidiaries in the UK, too.
I don't see any reason as to why this couldn't happen, the EU has already demonstrated its concerns on relying too much in certain companies, and they don't have as much of a flagrant issue as the aforementioned agressive move would imply.