T5A and B are opposite ends of the terminal. There's also a T5C which isn't open yet. Hence a lot of departures are from gate A10, the bus gate where F passengers are crammed into the same bus as the rest of the sardines.

BA's crew and aircraft are generally nicer but there's a nasty catch, If by the time you reach the front of the security line there's less than 35 minutes till (scheduled) departure, the BA goons will offload you - no exceptions. No matter that you can see the gate and there's no plane there yet. You will be offloaded.

I have a trip to BUD in April. Our TA recommends DFW-LHR on AA, LHR-BUD on BA, with a 1:40 connection in LHR. In the past, I've had tight connections in LHR with the T3-T1 shuffle, and managed to get LHR security to let us go ahead of others in line in order to make a flight. But you say there is an additional worry if you don't get out of security within 35 minutes.

I wouldn't risk it... Why not BA from DFW-LHR and stay airside at T5?

I'd like to stay on AA so I can use VIPs to upgrade to F. He ended up recommended DFW over LAX, though: DFW-LHR on AA, then LHR-BUD with 1:40 in LHR, which still sounds a little tight for the T3-T5 shuffle.

Separately, on a BRU trip, I have an award booked LAX-LHR-BRU all on BA, with outbound in J and return in F, but am toying with the idea of switching LAX-LHR to AA in F, giving me the better AA F seat over the BA J seat, saving a T4-TBIT change at LAX with extra security screen, but costing a T3-T5 change at LHR with security screens, busses, etc., plus a few extra miles.