Perhaps I've been watching too many spy dramas, but to me, Russian nerve agent being used to blatently target ex Russian spy screams of a set up from somewhere.
USA are embarassed by the increasing evidence that Russia was able to influence their democratic election.
Forcing their 'closest ally' to condemn Russia for an attack on British soil (and therefore condemn Russia) seems like a way to bring the world back to seeing America as top dog.
Why such an immediate jump to the obvious answer?