This is where the question of what would happen at MAN, BHX and STN with LHR and LGW full up is really relevant --especially as there must still be a fair chance that's what we'll end up with.
Indeed, but don't forget AMS, CDG, FRA. With LHR is full, carriers that can't get enough slots there do not necessarily end up in the waiting room at LGW. Some do, others go to LHR's competitor airports.
Most carriers flying to BHX and MAN tend to be at LHR already.