Round The World
All Ahead Full
Have you considered a Round The World ticket? Fly outbound via Hong Kong AND Singapore, then from Sydney you could fly via Hawaii and the USA, Mexico or Canada. You must not 'backtrack' and can have quite a few layovers. The cost of the ticket will quite possibly be less than the London - Sydney return. It certainly used to be the case that the best value in ticketing was a Business Class Round The World ticket.
Whatever you choose, have a great voyage.