I would suspect the deal will be conditional on certain domestic city pairs being retained. Government will not allow IAG to remove links to the regions from LHR, it isn't what the UK needs right now.
That said, if the Shuttles to EDI, GLA & MAN are upgauged to bigger aircraft as demand requires then bmi slots to those cities are fair game for going to long haul IMO.
Belfast must be profitable for BD or it would have been pulled by now, no sentiment was afforded to GLA. I flew with them just over a week ago and there were very few seats to be had out (A320) or back (A319). Perhaps IAG will just let it tick over as is while maintaining a close eye and getting the best of both worlds by enjoying feed from BFS and BHD?