London-Australia is a difficult route to operate if your hub is at one end of the route, rather than at one of the stopovers enroute.
Cultural/historical links aside, it is telling that BA and Virgin are the only European airlines to operate to Australia.
I think if BA had the choice it would pull out of Australia altogether and transfer passengers to Qantas at BKK/SIN, but WW has said that BA needs to serve SYD if it is to be seen as a global airline.
Virgin launching adding HKG-SYD to its HKG route made no sense commercially and seemed to have been done for reasons of vanity more than anything else. They cannabilised their loads on LHR-HKG as a consequence.