Pan AM and QF both used to have direct trans-pacific services with the SP although Pan Am's westbound service more often than not had to stop in for gas at HNL as the Pratts were not good enough fuel wise to go the distance in comparison with the Rollers on the QF SP. It started to cost Pan Am dearly in terms of both dollars and reputation.
Can't give you a start date to London but I remember going via BKK in a QF -300 sometime in the late 80's ('87 perhaps)?