The down side of course being that if anything goes horribly wrong, my parents are in Australia
Mate. You have a massive dilemma on your hands and it's depressing to think about. Mainly because it reminds me of when I was 11 and my dad had told my mum that he'd seen this amazing well-paid job going in New Zealand, which would of course have meant relocating, starting a new life etc. Mum got really sick thinking about it but when he came home he said he wasn't serious.
If you stick about, you gotta find people to fall back on if shit happens. Also you'll probably go aeons at a time without seeing your folks and that could include christmasses, etc. If you move with them, you'll have to make a whole bunch of new friends. Might be fairly easy if you have the "oh, you're from England, awesome" thing. It might be impetus to move to your own place there eventually.
Who knows man. Something good would come of either choice, I'm sure.