Many people coming to Australia are in the same boat in that they either cannot sell their house or do not wish to sell at a depressed price due to the current UK housing climate and will be renting out their UK property.
If you are in this position and the equity in your UK property would have been the deposit for your new home in Australia then a consideration is to release some equity from the property whilst in the UK before migrating which then can be used as a deposit here in Australia.
I have had meetings with quite a large number of people who are in this position and unfortunately it is very difficult for them to release equity from the UK property once they move to Australia. There are very few lenders that will entertain lending to an Australian resident.
If this is something that you would consider you should understand the implications of doing this before you go ahead. Especially in relation to how it might affect your borrowing ability when you get to Australia.
All Australian lenders will want to know what liabilities you have and your UK mortgage/payments will have to be taken into account. However some Australian lenders will not take into account the UK rental income (some will) and therefore this could reduce borrowing power.
Anyway certainly food for thought if you are in this position and would like to be able to buy when you are in Australia before the UK house sells (or you can save a deposit, 5% is the absolute minimum here in addition to stamp duty, total approx required is $40k on a $400k property).