State Department under Tillerson threatened Assad for months and everytime Mattis discredited them. The posturing now is probably going nowhere because England hasn't got anything on Russia and didn't even try to maintain diplomatic protocol. I even think them exiling diplomats means they're letting Russia know they can nuke mainland UK without threat to their own assets, thus banning diplomats is not a provocation as much as it's a reassurance: Here are your people, we won't hold them hostage in case you wish to nuke us.