Thing is that brexit UK would probably have to seek closer ties with Russia down the line, as relations with the eu worsen they would probably just take what they can and the Russians would probably be the only ones to offer them anything remotely favourable in terms of trade. A non EU aligned UK would probably seek to undermine the EU very much like Russia does now.

Russia has actually already succeeded in its plan to undermine the west with the brexit vote and the only way to change that would be to reverse brexit.

A brexit UK would eventually become a bigger liability to the west in its present form than if broken up into separate countries.