I think it really comes down to what Kaz says: What sort of life do you want?

You seem to have some serious financial obligations in the UK and moving to AU now may be very risky and could actually lower your standard of living.

I think based on your current info I wouldn't move at all. First you need to seriously do something about the debt (even if it takes a few years). Then after you've done that re-do the sums and see what is offered out here.

You should ideally come out here when you've eliminated most if not all of your discretionary debt, have some savings (first few months in AU use up funds during your setup), and you have a solid plan to be out here such that:

1. You improve your standard of living
2. Have identified what will make you happy by being here

When I came out here 5 yrs ago my circumstances were:

1. No personal debt
2. Late 20s age
3 5 yrs work experience in my field
4 Intra company transfer
5 AU economy doing well
6 Reasons other than money to come to AU, i.e. climate, beaches, wanted to do an expat position for a while, my skillset was needed here.

You're 21 now, I would plan to move here when you are between 23-25 if you've gotten rid of the discretionary debt by then. That will also let you get some more years experience and really define what you want to do in life and how you want your life to be.