Originally Posted by Globaliser

LHR-HKG and LHR-SIN are different zones.

You can now book to and from different zones, eg LHR-HKG / SIN-HKG. The rule prohibiting that has gone.

But once you have booked LHR-HKG / HKG-LHR, you can't change the HKG-LHR to SIN-LHR, because you would be changing zones to do so. The rule that prohibits a change of zone when you change the ticket is (sadly) still very much alive and well.

If you're prepared to route through the cities suggested by Swiss Tony, that would be a good alternative strategy.