Nothing, obviously. Their disinformation campaigns of the past 15 years have failed and the priorities have shifted towards terrorizing and misinforming their own citizens.
Maybe Russia funded some campaigns, not sure. Maybe they were able to sway some voters, again, not sure. But my impression is that there was already a sizeable portion of the population pro brexit. I'd be more inclined to lay this at the feet of Murdoch than Putin.
https://en.wikipedia.org/wiki/Russian_interference_in_the_20....