Queens Building was the original central area terminal, but I don't think it was ever called Terminal 2; T1 was built to the north of it then T2 to the south, then QB became airline offices, crew reporting and flight clearance/AIS etc.
When I started with BOAC in 67 there were two terminals, Europa which became T2 and Oceanic which became T3 departures. The Queens Building was the BEA ops centre, also the entrance to the roof gardens which stretched across T2 and to the 'corner' where you could get a view of the nearer T3 stands but not the further ones. The next central area buildings were I guess the BOAC cargocentre just to the west of the tunnel entrance - I was working there by late 68 - then T1 which probably opened in 69 (at which point the other 2 were renamed/numbered) and also the T3 arrivals building, around the same time.