As another poster said before, don’t actually think that IAG - BA or An other - can use these slots to fly Belfast Gatwick, as the slots were taken away from EI at BHD in the first place to enable the EC conditions on IAG’s takeover of EI to be cleared, and that condition remains in place. But I stand to be corrected on that if someone knows different?
Realistically Belfast to London is well served - one logical carrier flying to Heathrow (IAG), the biggest airline at LGW flying there (EZY), the same to LTN (EZY). The biggest airline at STN now flying there (FR). You could argue the only two oddballs are flybe LCY (rather than say BACF) but flybe are a much bigger presence in Belfast than BA, so sensible enough. And EZY to STN, but EZY have been flying this route going back to ‘Go’ days and they are the biggest brand in NI, so don’t see them giving any quarter on BFS-STN just because FR have finally made an appearance on that route.
Anyhow, I don’t think anybody should be getting too fretful. The London market is well covered as it is.