IAG have the monopoly on the Belfast to Heathrow route. They were forced to give up Gatwick. If a competitor wants those slots they have to be provided. IAG cannot use them on this route, however can use the slots for other routes until a competitor requires the slots to fly LGW to BFS/BHD. Nothing to do with Glasgow. Likewise a competitor cannot request the slots to fly another route. Akin to flybe and their Heathrow operations. EDI and ABZ only.
IAG could of stayed on BHD-LGW if they wanted to, yes they gave up slots to enable another operator operate a commercially viable alternative but they were free to continue BHD-LGW had BA wanted to give EI slots to do so or EI re-apply for new slots.
There was no obligation for them to pull BHD-LGW. IAG took a wise decision to let it go for the greater good.