> I'm frankly surprised that Sheryl Sandberg isn't being sent to the UK if Zuckerberg is testifying in the US (although I don't see why he can't do both). The whole angle at the time of her hiring was that she would be "the grown up in the room".

Arguably, that perception is exactly why he has to send a clear subordinate if he isn't going himself, and not someone perceived to be his babysitter.