If you're doing the two sectors back to back then I'd be using it on LHR-HKG as well - my strategy is always to get on to the timezone of the destination as son as I get on the first plane, so if you are taking an evening departure from LHR I'd be trying to sleep during the day preceding and then take breakfast as the first meal on the plane (morning Sydney time), stay awake (AVOD, etc) then take dinner a couple of hours before you land. Sleep HKG-SYD (maybe quick supper after takeoff) and arrive in the morning.