I am heading to Sydney soon and have booked BA from LHR to SIN in PE, and then taking Scoot the rest of the way. Given that BA wanted close to £2300 for a return PE fare, and only £1200 to SIN return, plus around £600 return SIN-SYD-SIN with Scoot in ScootBiz (which I believe is roughly equivalent to PE) for only a slight inconvenience I am saving £500. It is another possibility for you perhaps......
Or try AF's Premium Voyageur to SIN via CDG then try ScootBiz. Another option I looked at was KLM to KL and then AirAsia onwards to SYD.
Hope you find a good option.