What a fascinating thread.
Heathrow Director: The Northeast Trident doing a "split-arse circuit" at LHR could well have been flown by the late Capt. Arthur Whitlock. He mentions in his autobiographical book "Through the cockpit door" sitting in the right hand seat of a Trident at Hatfield flown by the late John Cunningham, who performed the same procedure, as part of his type conversion training on the Trident.
I can recollect BKS using the call sign "BKS" followed by the aircraft reg. in the 1960's. For example, "BKS lima lima" and "BKS bravo kilo" were very commonly heard callsigns on Amber 1 and Amber 2 airways in the mid to late 1960's, and belonged respectively to Britannias GAPLL and GANBK as they shuttle between LHR and Newcastle.
I can also recall that even some major airlines used the aircraft reg. in the 1960's as opposed to flight numbers. Sabena did for all flights, even transatlantic 707's, and Air France did for all European flights (but not long-haul, except for charters). BEA of course used the prefix "Bealine" followed by the aircraft reg. BOAC however always used "Speedbird" followed by the flight number.
Happy days!