There's squillions of threads floating about on this forum that deal with conversion from down under to JAA licences. Do a search for posts by Redsnail or myself, there are several long posts that spell it out in detail.
In brief, you'll need to do :
JAA ATPL theory course of training + exams (about 3,000 quid)
Whatever hours you need to get minimum 200 hours, including 25 hours of compulsory CPL dual training + CPL flight test (5 or 6,000 quid + hour building)
If you have no instrument rating, then you'll need to do I believe around 60 or 70 hours of compulsory dual training + IR flight test (10 - 12,000 quid)
Although it's not compulsory for licence issue, if you want a dog's show of an airline job you'll need to do a Multi Crew Cooperation (MCC) course (2,500 or so)
Suggest you at least try to get the Oz CPL and IR issued before you head over to the UK, it will save you a lot on conversion costs. Needless to say, in the current climate jobs for freshly qualified CPL holders are as rare as rocking horse sh!t so make sure you pick a UK school to train at, that might let you hang about and work as an FI until things pick up a bit.