I have done this quite a few times int he past and the cheapest way is the Excess Baggage Shippers.
A quick check online shows this for shipping by sea with the first mob I looked at (
https://www.sevenseasworldwide.com/):
Date of Quote: 12/04/2017
Collecting From: Sydney, Sydney Streets, New South Wales, 2000, Australia
Delivering To: St James's, City of Westminster, Greater London, SW1A, United Kingdom
Transportation: Sea
Service: Door to Door
ETA: 68 - 84 Days
Storage: 0 Weeks
Shipping Price: AUD169.38 - Plus some form of inspection fee
A AUD99.00 Deposit is payable on order, the remainder (AUD70.38) is then paid after your bags
and boxes are collected in Australia but before they are dispatched.
Max weight per box/item: 30Kg