My understanding is that the crews swapped aircraft ie BOAC crew flew the QANTAS aircraft on to London, and the QANTAS crew flew the BOAC aircraft to Sydney. That seems to be confirmed by the above photo of a BOAC aircraft parked at the QANTAS Sydney facility. BOAC at the time was a major shareholder in QANTAS, which was relinquished when QANTAS was nationalised.