Originally Posted by
Ldnn1
That's not correct. You can call BA and they will do this all on one booking for you, assuming availability of course.
(I read the OP as wanting to pay for both flights with avios, but combining F+Y, which cannot be done online but can over the phone. If OP wants to pay for the Y flight in cash, then no that wouldn't be possible.)
I don’t think so. OP wants to redeem one-way LHR-HKG in CX F and then buy a one-way Y to MEL. To my knowledge there is no way to do this on one booking, but CX may choose to check through. Protection across two separate tickets isn’t going to happen though.