Rain and flooding IMO effects the countryside more than big cities which have the infrastructure to handle the run off. The vote to leave the EU came from outside the big cities. If anything the vote to leave could have been larger if weather had any impact at all.
When the weather is bad, it’s the less passionate voters that stays home. Assuming those who didn’t give a damn enough to brave bad weather stayed home, it probably leads to a more honest vote dominated by informed voters with more than a casual opinion.