Just a few comments:
- Keep in mind that the ticket is valid only 1 year after the first flight. And the first flight has to be not later than 1 year after you purchase the ticket.
- SYD-LHR always requires Asia as a continent. You could do North America-Asia (few flights there) - Australia - LHR. Or just stop in Asia for w hile on the way back from Australia without extra cost (except the taxes).
- I usually buy AONE and never had problems finding seats in First on similar routes with one or 2 days flexibility and occasional wait listing.
- Check the rules. I think there is a limit how often you can touch the same city (LHR/LGW) and also a limit how often you can leave a country. I was running into this before.
- You are getting close to the 20 flights limit. Surface also counts. And the return flight to Africa also counts.