Earlier quoted context omitted.
Even if the EU did not have jurisdiction British authorities are not very keen on large mergers either. British resistance caused American Pfizer to walk away from a potential merger with British AstraZeneca.
I'm not sure how much sway British authorities would have, since ARM is now wholly owned by SoftBank?
There are countless examples where country 1 has stopped a merger between a company from country 2 and one from country 3.