Reference flights on BOAC during the war years, I doubt if it operated much if at all as a conventional airline (ie carrying private fare paying pax) on any of its routes. It was essentially a nationalised civilianised Transport Command, carrying official (ie Govt sponsored) pax/cargo, but often to neutral countries where military flights were excluded. How the seats were sold I have no idea, but would suspect that the payload offered was chartered by HMG. There would thus be a system of 'offering space available' carriage by some form of Air Booking Centre. So if we had a crew etc to move from A to B with spare seats available then they would be boarded rather than let the flight go part full.
That's my theory anyway, but merely based on post war Transport/ Air Support Command practice only. Ready to be rigorously corrected as ever...