Brexit benefits the Russians.
Firstly, it divides a large bloc (the EU), and divide and rule of anyone who might stand against you is always good.
Secondly, it throws the UK into paralysis, as we're currently seeing, where we can't even pay attention to our own internal legislation and issues, because everything is now about the logistics of Brexit. There's a similar though lower cost to the rest of the EU.
Thirdly, lots of dissension, and dissatisfaction and disruption in a major country... if they can't find pickings of one kind or another in that, they're not the militarised, aggressive superpower I take them for.