I believe the lounge was on the roof when that area used to be accessible landside, I have no idea what that area is used for anymore (probably offices). The lounge does occupy some of the area which was used for the original expansion of the main departure lounge.

At one time departures area (the public departure lounge, gates, shops) all used to be on the ground floor, first floor was the landside bar and cafe (and at one point a table service restaurant and briefly a Dan Air lounge) and second floor was the rooftop viewing area with extra cafe that never seemed to be open where they then built extra offices up there, some became airline lounges.

As there wasn't much room on the ground floor they extended the departure lounge area outwards (making the view in the cafe upstairs less attractive but the capacity increase was definitely needed). From this departures lounge you now had a similar view of the gates as you now do from the BA lounge but originally those gates were international gates and the domestic ones were the other side.

There was a huge gap in me flying from NCL at this pointso I don't know quite when departures moved upstairs allowing the BA lounge to be where it is now.