From the outside, I can’t see a single positive repercussion of Brexit.
The timing's turned out pretty un/fortunate (depending on your point of view; regardless of political side of it) with the coronavirus - complicating what's due how much to which, whether either can be blamed, affected the ability to respond to each other, etc.
there is absolutely no way that undermining France would have been on the table when the UK was part of the EU common defence