I am sure if BA are offered the right price they will open their facilities to anyone who wants to pay! The amount of premium trafic currently using T3 is pretty low currently, and also the BA lounge is pretty big, so could easily be patitioned.
T3 is set to undergo some big changes. Obviously MAplc recognise that expansion can really only feasibly occur at T2 and T3 so I would suggest that T3's days of being a BA bastion are possibly numbered. Watch this space I would say!