I take it that from your post that this hint is alittle more than speculation?
No I'm afraid it is pure speculation, just who I personally think are most likely candidates.
Aer Lingus is a fair shout, although I think they only have 2 A319's so not sure if they'd be easily available. Although I suppose if its makes them money they'd make it happen. Also in EI's favour is the fact that VS LHR-MAN will be using T1 (not T3 as VS's other flights), so will be handled and checked-in by non-virgin staff. As EI are already in T1 that would make things easier to manage...Imagine we'll find out soon