...over his support for Assad*; the Skripal case; other suspicious UK deaths of Russian (and British) citizens; and Russian meddling in the US election and in Brexit.
Basically, 'don't push us too far'.
I suspect the US is leading it and Britain was 'encouraged' to take part, just as the US is placing economic pressure on the UK to fully support the Russian sanctions.
*I happen to think the West supporting the Syrian opposition was absolute madness.
This is interesting in context: