Definitely have a look at CX. Also consider how much you could almost certainly save by buying their tickets to start from continental Europe rather than the UK. For example, I think you would find that buying an Easyjet/Ryan/whoever cheapo ticket to Amsterdam/Paris/Rome and then taking CX to Aus from there via HKG would be significantly cheaper than buying tickets to start in the UK which is one of the most expensive countries in the world for airfares.
As an example, CX offers its "Pass Asie" from Paris for 1450 Euros (960 GBP) which would offer round trip CDG - AUS with stopovers each way in HKG PLUS a side trip either on the way out or the way back to any destination that CX or KA fly to from HKG. There is full flexibility to change dates / times of flights. See (in French):
http://www.cathaypacific.com/fr/offe...-44129,00.html
(ignore the validity date - it was still available when last I checked a few weeks ago)
Consider also a round the world ticket which can sometimes be had for not much more than a round trip to Oz, and which could give them the opportunity to see many more places.
I have always found Trailfinders (
http://www.trailfinders.co.uk ) to be pretty good at sorting out long haul deals.
Note that you will never find tickets with a validity of longer than one year so you might need to do it in two halfs somehow.