Originally Posted by Johnosan
1. I'm mad
2. 2374 miles
If they would allow 2 transits on the same day it can be done. Night flight out of SYD - HKG and HKG - SIN during the day night flight from SIN - LHR
If collecting miles is important, then there are better ways to get them than this route.
E.g. if you replaced SYD-HKG-SIN-LHR with a simple SYD-xxx-LHR, then you can use your two remaining Asia segments earlier in your first Asia visit (i.e. before getting to MEL), and get more miles. NRT is included below because of the F availability which changes it from simple comparison.
HKG-NRT-HKG: 3684 miles
HKG-CTS-HKG: 4264 miles
HKG-DPS-HKG: 4298 miles
You also get the added benefit of a simple routing which you won't have to argue about whether it's valid or not. This factor could be especially important if you need to reissue somewhere. And you will also use your 4th Asia segment, which you haven't done.