Not quite, on the landside of T5 arrivals there are signs that say "Airline Lounge - By Invitation Only".

However the "invitation" is extended to all F and CW customers as well as Gold/Prem in all longhaul cabins. It's just to prevent people from going there from curiosity,
e.g. "Hmmm...while I wait for my friend Johnny to arrive from Paris, maybe I should go see if I can wait in this airline lounge thing they have upstairs. Maybe it's nicer than these plastic seats."


Back on topic, despite the fact that HIDDY correctly states the "official" public rules, I've never been denied checkin, fast track, or boarding together even if there are separate bookings, separate classes/status, etc etc.