See https://www.flyertalk.com/forum/brit...l#post33726938. It is a recent story of someone who should have been at a B gate, accidentally went to a C gate, and then took the shuttle train back to the B gates. By doing that, he crossed from the departures stream to the arrivals stream. He needed to re-clear security. He ended up back the main terminal (where the A gates are). By then, he had missed his flight.

Even if he hadn't missed it, his existing boarding pass probably wouldn't have allowed him through security again. He would have had to get a new one.

For anyone who really knows their way around Terminal 5, including where all the lifts go and how to walk and not take the train, none of this would be stressful. T5 has been around long enough that many of us know it as well as our local shopping centres (if not our own homes). But if you don't, T5 is full of elephant traps.

Consequently, I share the advice: keep it simple, and stay in T5A (the main terminal) and use the lounges there unless you have a confirmed B gate number.