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.)
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.
Microsoft and Activision do both have various subsidiaries in the UK, too.