T5B. No contest. Just make sure you walk there and walk back, from the bottom floor of the lift. Travellators speed you there, and an ok pace, its 8 minutes back. Just do not use the train.

Only real caveat to note is the escalators just go up at T5B from the tunnels. But you take the lift back down.

The screens also do not show your departure gate if it is at A, so you need to keep up with that via your phone or go back sufficiently early to be sure.

We did have a weird experience once with T5B. It was busy! Then they called the Barbados flight and we were back to us two and one other...