Californian cities (LAX/SFO) are good for schedule efficiency as the round trip is slightly over 24 hours, the aircraft typically returning to London just after the next day's flight has left. If the aircraft that does a mid-morning departure and return can then be assigned to an after-lunch departure to the other city, this fits in nicely with the early morning arrival and evening departure of the Johannesburg flights, with comfortable turnrounds at London.
On a historical note, this is exactly how the first BA 747-200Bs, with enhanced range, were deployed when they arrived in the fleet in 1977.