If we can include time travel lounges then I recall the probably first BA Exec Club lounge over at T1. A couple f repurposed offices above the BA shuttle check in desks that were at the far left of the hall and set back against and blocking what was a magnificent picture window of lines of Tridents and 757s.
A punch key pad allowed you entry to the exclusive world of Exec Club where a bunch of Thistle Hotel-style lobby chairs pushed against the wall awaited you. A hotel room fridge with booze, a coffee on a hotplate and a TV with rolling Teletext LHR Departure info.
It must be said that BA then were the pioneers of airline loyalty taking it to a level way ahead of all the other carriers. Always expensive but never mediocre.