It's broader than that. The CMA considers impacts of mergers (and other competition-related concerns) on UK markets regardless of where an entity is headquartered. It has broad investigative and enforcement powers over any entity selling in UK markets with over $70M GBP in revenue from UK markets or 25% of market share in a given vertical. If Microsoft and Activision merged without making accommodations to satisfy the CMA, the CMA would be able to block their access to the UK market and fine them.