To anyone saying that it's nothing to do with BA, I think where it could be relevant is where BA planes are scheduled to land anywhere in the EU at about the same time as a bunch of other UK flights. Because all other things being equal, these are going to be the major congestion points to watch out for/avoid in future.

I recall a number of years ago being on a VS flight to one of the Caribbean islands, and the pilot was making a thing in his announcement of the fact that they'd wangled a take off slot ahead of the BA flight around the same time... and that we'd get the benefit of a quicker route through immigration. Needless to say we did but ended up waiting an age for our bags, but there's a nugget of truth here if our brave new Brexit means overall longer waits in the passport queue...