I don't know why, but this sort of behaviour by any "celeb" makes my blood boil. And the effect is worse when BA (or any organisation) fawns and panders to them and actually gives in to them. Why do organisations do this - is one of lifes many mysteries. Like all of us, they are only entitled to a seat in the cabin they paid for. Any specific seat should only be decided by FF status or when OLCI opens.
+1. If I was asked nicely by them, I'd consider moving, but if they acted as described by the OP, I'd downright refuse to move if asked, and would only do so if explicitly told I had to by the crew. I'd then be writing to BA in the strongest terms, and naming the DYKWIA them via all social media outlets available to me
Pandering to them like that just encourages their childish behaviour IMO.