Originally Posted by
phlwookie
Just to confirm other replies, this morning someone ahead of me at Fast Track security in LHR T5 was denied because their boarding pass was out of another terminal. They asked to be let through to use a T5 lounge, but the employee told them to access lounges in their departure terminal. I can't imagine it'd be any different for T3 with a T5 boarding pass.
This is a rule established by the old British Airport Authority, and it still exists under the Spanish construction company Ferrovial.
In the days when arriving passengers mixed with departing passengers at T-3, passengers were screened to assure they were departing. Those not in possession of a T-3 boarding pass for departure were denied lounge access (even Admirals Club members).
Thanks for the update; members' experiences are invaluable for a current knowledge base.