Well, they have already been caught (twice?) assisting, Germany in discussions with Ukraine about how to do it and make it look like Germany wasn't invloved... but that leaked. and then again I think (can't recall details). Also UK foreign minister said it was Ukraine's right to use UK weapons to strike inside Russia and analysts expect that is the target.

This being said, I think Russia wouldn't go so far, and I would take it as a massive escalation. But not impossible. And Russia has been very restrained about anything outside of Ukraine and I half expect when they break that restraint, it will be a sudden significant break of restraint.