Not that long ago, I was reading (I think on the BAA website) that it's actually H M Customs and Excise, or/and H M Immigration that draw the rules up so only passenger departing from the respective terminals can use the lounges. Not an easy site to navigate, but I know I rad it someplace, it's a government issue, which is why they sometimes have a BAA emplyee vet people before yuou even get to the door of your favorite (or other) lounge.
And you are quite right about the BA "print your own" problem, as verified by some of the BA flyers herabouts. And, the lounge regs generally state, like AA's, that one should produce 1) the card, 2) identification. Not BP, the card.
Originally Posted by MIAFlyer
It is my understanding that according to the BAA (the operator for LHR airport) that ALL lounges (AA, BA, LH, UA, etc.) are available only to passengers departing from that terminal. So, while you could, in theory, physically get to whatever terminal you want, none of the lounges SHOULD let you in, unless you are departing from that terminal.
Don't ask me why, but I do know they are very strict on this. Check some of the postings from the BA forum...
As for the requirement to actually present your AA Plat/EXP card, this is because BA's self check in system allows you to put whatever OneWorld frequent flyer number in the record. It then allows you to put whatever status you want as well. So in theory, an AA Gold person could go to the self service check in, put in his AA number, and select EXP as the status, and it would print a boarding pass with Emerald on it, and try to access lounges he/she is not entitled to access.
Have fun!