AerLingus has been one of the obvious contenders for this, given the size of their operation at LHR and their proximity to BA, lets say. (interlining on ORK, SNN, DUB and BFS to LHR, which is important to BA and EI alike).
The other likely candidates can only be to my mind
Virgin
bmi Regional
Virgin very unlikely to be serious about doing domestic.
BA would much rather have EI in the fold as they will have to cooperate anyway, as per the conditions of allowing another operator to operate LHR EDI and ABZ, if EI dont go for ABZ I cant see any other operator going after it.
Perhaps as a concession, BA could opt to serve INV in place of the slots to be allocated to ABZ route.?
As much as BA has made strong signals of its committment to BHD and the Belfast market (all very welcome), I still feel that EI doing a deal at BHD is linked to some sort of unwritten agreement with BA where EI gets onto EI ex LHR and BA exits Belfast martet. Whether EI is successful in getting LHR EDI slots that remains to be seen.