The tech stops via Winnipeg (and sometimes Calgary ) were also to change cabin crew.
Believe it or not-BASSA union rules in those days would not permit the cabin crew to continue-but the flight deck crew still could and did-whereas the cabin crew slipped.
So even if a flight could make it direct from London-due to tailwinds/light loads etc-it would still have to stop to make a cabin crew change. It was just one of those crazy union driven agreements that prevailed in those days-that went a long way towards making it a major financial loser-but nobody seemed to care then-as it was government owned?