LHR-HKG-SYD would be 180.
LHR-SIN-SYD with a flight number change would be 190, as LHR-SIN is an extra long route.

I suppose if you want to do HKG one way, you could do

LHR-HKG-SYD (180)
SYD-MEL (10 in the correct booking class)
MEL-SIN-LHR (190)

Edit: orbitmic beat me to it...