It's not cheap!
The Oriana, which is the only cruise ship to occasionally ply that route, departs from Southampton on 23rd September and takes 41 nights to get to Sydney (via Madeira, St Lucia, Margarita, Curacao, Panama Canal, Acapulco, San Francisco, Honolulu, Apia, Auckland and Wellington) with fares starting at £3,989 one way.
The return journey is routed via Brisbane, Whitsunday, Manila, Hong Kong, Phu My, Bangkok, Ko Samui, Singapore, Port Kelang, Mumbai, Sharm el Sheik, Port Said, Piraeus and Lisbon, taking 44 nights, with one way fares starting at £4,044.
(You'd still have to get from Sydney to Melbourne as well).
There are a very small number of freight service which carry a few passengers, with fares typically around £90 per day. That sounds quite cheap until you realise that, with journeys taking several weeks, a one-way fare still comes to several thousand pounds.
Chris