Agree with Poppa Jo, QF has minimal operations to Darwin compared to Perth, so they already have a heap of flights going to Perth, which could feed the PER LHR service.
I recall reading that Qantas did a survey sometime back, of top tier regular International passengers, when the 777LR was first available re a non stop Sydney London Sydney, and if they would pay the premium.
The most interest was in a non stop London Sydney, they were happy to transit Perth on the way over.
Would the 787 have the range for London Sydney non stop?