The only airlines serving Belfast with the Union flag on the livery whatsoever are;
British Airways - tail fin
Virgin Atlantic - below cockpit
Thomas Cook - beside registration
Thomson Airways - beside registration
So unless Keddie meant it was a british airline in general, in which case Jet2, Flybe and Eastern Airways can be added to the mix, it is one of the four airlines above that will operate the island's sole service to this new destination.
Keddie said the Union flag is on the tail, and not specifically the tail fin, so I suppose Thomas Cook and Thomson could be the contenders. The registration (and flag) for both of these airlines is located in the tail area, if you were generalising.