Unfortunately this is booked with Asia Miles so their policies apply, not necessarily BA's policies. If it was a BA 125 ticket then BA would normally be ok to just delete the GRU sector and refund the Avios, but I've no idea what CX's approach will be here. I would not be confident that LHR-SYD will open up any time soon, I think we may be over a year away for that to be viable.