Airside, again you are well within MCT. Don't panic when you get to T3 Flight Connections Airside, it may well be heaving since vast planes all seem to head East at the same time. Despite the apparent queue you should be fine for time, it looks worse than it is. If you are one PNR then BA would expect you to be going airside, in the unlikely event that they feel like rescuing you from the queues. If the lounge experience is part of the travel experience you could book the earlier flight, but I wouldn't bother.