im trying to apply for UK uni's now as well. its a bit easier to understand than the US system, but its a bit weird as well.
To get into the prestigious ones (as mentioned above), you do need a high UAI. Cambridge and Oxford state that Australian students need approximately 98.5, in order for their application to be considered as 'effective'. Other, really good ones such as Imperial College London, London School of Economics and Political Science, University College London, St Andrews etc, you need around 95. With Oxford and all that you have to apply specifically to a college, which is a bit confusing, but there's so many and you just pick one according to what you like etc.
when do you apply? to start uni there in 2009 (you dont need to worry about it now) you need to hand in your apps by October the 15th, i think? just have a look at the UCAS website; their version of UAC site. so the application date should be around the same for next year. i dont think that your UAI gets converted, instead they have 'specialists' who know the education system of other countries and they determine the applicants through that. you might get called to do a phone interview as well.
thats all i can think of at the moment :S hope it helped