This was almost certainly done by American embassy staff under cover. It’s thus a state, not criminal, issue. If Britain wanted to eject American diplomats for spying, it could do that, but...why?
It's still a criminal issue under UK law, in that the law was broken. Obviously, diplomatic staff cannot be prosecuted but that does not negate the fact the law was broken.
This was almost certainly done by American embassy staff under cover. It’s thus a state, not criminal, issue. If Britain wanted to eject American diplomats for spying, it could do that, but...why?