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.