The runway at City isn't a constraint for BA; look at their Dublin pseudo-Shuttle* service, they could put any aircraft they want on it but they go for frequency with the A319 / 320 / 321. There was much excitement at Christmas when it was rumoured they were scheduling a couple of B772 rotations into Dublin but they turned-out to be A321s.
* it's about time they dropped the Shuttle moniker in general, it has no connection to the original walk-up-guaranteed-seat service.