OK, i've made it back to the Main terminal (Gates A) from the T5B... It was actually quite simple.. and no need to secretive about it. There's a big sign pointing the way to GATE A... which is behind the elevators.. There's a security guy manning the entry way, and he escorts you downstairs to the basement and takes you (by golf cart) from the Terminal 5B to the main terminal (about 1 mile) and you take the elevator.
I asked him if this happens a lot, and he says "plenty, and it only gets worse next week when more flights are moved to Terminal 5."
I asked him why BA doesnt install moving elevators for people to move back and forth between the terminal, and his simple response is " we've got no more money."
So there it is....