The building that was BOAC's London Terminal near Victoria Rail Station is still there. IIRC it has a central tower with a 4-face clock and can be seen from trains as they approach Victoria Station, and can be seen from Victoria Street.
In those days, BOAC used Oceanic Terminal (now Terminal 3) at LHR
and BEA used Terminal 2 (and later Terminal 1). Both airlines operated double-decker buses with trailers from the London Terminals to LHR
. I think buses departed at 15-minute intervals during the day time. BOAC passengers could check-in at Victoria, and BEA passengers could check in at Cromwell Road. They would be given their boarding cards and their luggage would be taken care of until they arrived at their destination.
British Caledonian (and before them BUA) had an advantage over BOAC/BEA as their Central London Terminal was located immediately above the train platforms at Victoria Station. Passengers could check-in here, be issued with their boarding passes, relax in the business lounge and then take an escalator down to the platform to hop on a train that would take them direct to Gatwick Airport Station. The train journey to Gatwick was 40 minutes (later reduced to 30 minutes when the Gatwick Express started), whereas it took the double-decker buses an hour or longer to travel along the A4
/M4 to LHR
MOL on SRB's latest attack at BA: "It's like a little Chihuahua barking at a dying Labrador. Nobody cares."