6F is indeed correct. Late 90's the lounge was located upstairs at the far end of the pier, with BA to LHR using gate 8 (or maybe 7). Prior to this BA always used gate 10 right at the front (with Dan Air normally relegated to gate 9). The left hand side of the pier back then was wholly for domestic flights. Underneath the BA lounge was the departure gate you would use if flying international from one of the top gates (where they used to put the wide bodies that couldn't fit anywhere else).

IIRC, the lounge was relocated a few times but I do remember it was closed in April 1999 and BA used a contract lounge at that point.

In the 80's it was located on the first floor of the main terminal. I know that in the 90's the contract lounge was once located on the 2nd floor (i.e. the roof) but I never recall the BA lounge being up there.