Think most of the BA stuff was changed when BOAC and BEA came together, some time after 1974 when the BE prefix was dropped and the "Speedbird" callsign was adopted fleetwide.
Aer Lingus have some longstanding routes to the UK from Dublin that are years old and still going strong, I wonder if a QF001 might be up there? I bet it's a Heathrow service with number being "one".