1. Try a 2 airline RTW (was your original quote for some alliance?) where the two partners cover all legs. BA and QF (alone) would work. One such as Cathay Pacific with South African Airways might also work if it exists.
2. go in low season
3. go out of Canada (fares slightly lower than ex-U.S.)
4. Stop in London for < 24 hrs (your itinerary suggests a 4-continent i.e., N. America-Australia-Africa-Europe-North America. You might be able to cut out a continent that way.