Are you planning on flying in First, Business or Economy?

If you're starting in MEL, planning on seeing parts of Asia, then going to the UK and then returning to Australia, a oneworld Explorer (RTW) ticket may be better, rather than a Circle Asia & SWP ticket coupled with a flight to London and back.

The Circle Asia & SWP ticket is mileage based, so the precise routing matters. You can't fly SGN-MEL on a oneworld airline. QF has a codeshare on Vietnam Airlines but you can't use that on a Circle Asia ticket. Determining the precise routing thus becomes important for determining whether it fits in the 13,000 or 17,000 mile variations of the ticket.

For example, mel-bne-kix-nrt-sgn-hkg-mel is 13,809 miles so pushes you up to the 17,000 mile variant.

The oneworld Explorer tickets in First and Business are relatively expensive ex-Australia but the economy ticket is relatively cheap, so depending on what class you plan on flying, that may impact the optimum ticket combination. However, on that ticket, you would have to do all your Asia stops (i.e., Japan and SGN) consecutively as you can't stop in Asia again (once you've left the region the first time). That obviously may not work for you.