BA (and indeed all European Airlines) haven't flown to Australia for a while. It's all codeshare out of Asia. The change is that QF won't be continuing to LHR from BKK and HKG... BA is leasing those routes.
BA will still have service to Australia on their own aircraft with one stop via Singapore once this change is made.
BA is leasing those routes.
The routes are not being leased, Qantas is pulling off HKG-LHR and BKK-LHR entirely.