To put in perspective, if Alibaba's partners was caught collecting massive amounts of customer data through Alibaba and using it to spy (in part for Chinese govt) and perform experiments/tactics on the American people, would they first summon the highest American branch leader of Alibaba in hush-hush manner or the most notable founder himself/herself with the most voting power on changing and directing the organization?
UK isn't USA, if they feel that the liberties of their own people is being tampered with, they would want to talk directly to the big man, the UK-based CEO would rather be just a horn through which Zuckerberg and his team talks through so it would be more direct to talk to him.
Moreover whole situation is like a Catch-22, if Zuckerberg doesn't go, it would betray him more.