Booked J SYD LHR via HKD of course, AUD 78XX return, it is the same price if you do a s/o in HKG.
However, if you just book SYD HKG J, is about 6xx_, not so much difference, however you miss another 24 hrs of J time, and UK visit.
If you just book HKG LHR, it is about 47xx_ HKD, which is...