I have an upcoming LHR-SYD flight where I am flying from LHR-HKG with Virgin Atlantic and then transiting to Qantas for the flight to Sydney.

Here's the upshot of my trip:

Botth flights on same ticket so luggage was checked thru to SYD, no problems.

Whilst waiting for my VS flight at T3 at LHR I went to the QF service desk and was given my BP for HKG-SYD. This ended up being a smart move as it meant I didn't need to queue at the QF transfer desk in HKG which looked very busy.