Something else our Sydney-centric viewers can't seem to fathom is the significant passenger market that exists between Perth and London. According to some studies, the total number of people in Perth who were either born in the UK or have close family connections is as large as that in Sydney or Melbourne. At the moment, that market cannot fly to London from Perth with Qantas without backtracking to the east coast. Sure, they can take an Emirates code-share flight, but how many bother booking through Qantas when they can book directly with Emirates, or take one of the many other options that are available with other airlines via Asia or the Middle East? Do you not think that Qantas should do something to try and tap that market?
Yes. This is likely the key to a lot of it, and my personal opinion is that Perth alone will contribute a VAST majority of the business for this service. The UK Passport holding population of Perth alone is almost the size of Darwins total Population.
Add to that there are people who WANT to fly Qantas. They recognise that they have upped their game a bit over the last few years, and if they only flew services to and from where they wanted to go (i.e.; ex ADL and PER), then they'd go QF.