I don't know about part miles part cash, but there are a couple of questions.
The MasterCard earning rates are pityingly miserable. Even with the fee cards, you would earn just 1.25 Airmiles per £50 on the MC and it would hurt (me at least) to whip out the card for that rate.
Yes, BA fly to Sydney, so subject to availability, you could get to SYD. The flights would be in WT, and SYD in WT demands a strong constitution. At least, all charges and fees are included.