British Airways neither have enough aircraft to cover for their increased flying ex LGW let alone spare or enough cabin crew to cover them hence why they are using aircraft from the likes of Titan who will be using their own crews to cover the British Airways flights.
I've travelled with Titan on a B757 operating on behalf BA and in their equivalent of Club Class and I was very surprised by how good the cabin crew were, I couldn't tell the difference and the crew were excellent with the full BA club menu on offer, I was sat in seat 1A and I was first to board and I was even invited to the cockpit before departure after politely asking which I thought was a nice gesture but the pilot obviously knew I was an aviation enthusiast as we discussed the B757.