I will put everybody out of their misery and confirm that it was the BA First lounge at LHR T1. This lounge opened in c.2003/4 around the same time as the International Terraces lounge was refurbished and some longhaul services moved from T4 to T1. Sadly, the lounge only had a 5-year life before closing in 2008 when T5 opened.
Congratulations to hjd36 who correctly guessed LHR T1.