The UK doesn't need to block Facebook, they can just ban advertisers from buying any ad space on there. FB could still provide a service in the UK but it wouldn't make them any money.
So for example, Apple US couldn't buy ads on Facebook, because it shows ads to UK users? How do you envisage this working?
I was replying to this. If they can still show non-UK ads to UK residents then they can still make money.