I just posted this on another thread but this might be a better place:
I'm flying into Terminal 5 at LHR on BA (coming from ABZ) and have hours to kill before meeting a friend to get a long distance bus - which I believe leaves from the airport. I'm a United Club member and UA Silver so I'm pretty sure I can access the UA Club at Terminal 2. But can I get from 5 to 2 airside without an outbound flight from 2?