There were a lot of charters-in at this time, BA were notably short of aircraft, long-term, and the Trident problems just added to it. I can actually recall Britannia 737s at Heathrow operating BA subcharters in 1978, along with two MEA 747s which were leased for a couple of years.
However there was some crossover between the divisions, as two VC-10s were moved across to European work sufficiently permanently as to appear in the timetables, they operated to Athens, Larnaca, Lisbon and Amsterdam. I'm looking at the relevant timetable now !