Yes it was about the time they actually merged/changed name from BOAC to B.A.-around about 1974 if my memory is correct.
The 707s were usually the 707-336 versions. There were some strange union flight time agreements then and the cabin crew used to change in Calgary but the same flight deck crew carried on to L.A (Its usually the other way round and its the flight deck crew that have the duty hours problems before the cabin crew do)
London to L.A was also getting close to max range on a fully loaded 707 against the prevailing winds too-so would also possibly have been a combined en route fuel stop too.
The flights would normally come back direct from L.A non-stop.