They have the incentive of their own citizens being exposed to nerve agents, polonium, and who knows what else. I don’t think “if brexit” is the spur it was last year either, given the sheer number of backbenchers who seem to be maneuvering for another referendum. Russian trade isn’t as valuable to the UK as EU, US, and CA trade either. Over it all, China is getting much more cozy with the UK (Hinkley Point anyone?) while Russia is moving away.
The UK doesn’t need Russia, long term, especially with the rise of China as an economic force.