I don’t think you are in a weak position at all.
The law is the law and regardless of whichever tax haven their head office is registered in, FB operate in England therefore have to abide by UK law.
I’ve seen lost of posts recently about how they are misleading people, but because the individual amounts involved are small then most people aren’t complaining and so FB gets away with it. Combined of course, it adds up to a very healthy increase in profits for them. FB have been offering free credits for advertising but when you look further into it, there’s a minimum spend eg you have to spend 15 to get the free 10.
Maybe it’s about time the Office of Fair Trading were involved…