Hey Amelia,
I'm from Australia! I was born in Brisbane and moved back to go to uni there. I am also a secondary school teacher and was a head a year in the Queensland education system. I'm here to answer any questions you have.
It's definitely possible to move mid- A Levels, although my very strong advice is that you get in contact with Australia schools very soon to ensure what you study in A Levels here will count if/when you move. In the current Queensland (QLD) system, high school is from Year 7 to year 12. It's divided into 'Junior Secondary' (Year 7-10) and 'Senior Secondary' is Year 11 and 12. Although unlike a lot of A-Level colleges here, it's usually in the same school, you still wear uniform, everything is pretty much the same, except the curriculum. It's a two year course and it does have pretty specific requirements for what course work and time you need to have spent on your subjects. It does work quite differently to A Levels, in that there are less exams and more course work. You also have to study English and Maths - there are different streams/levels of difficulty, but it is compulsory. You also normally study six subjects if you want to go to uni.
Uni isn't that different to here, although, as others have said above, the entry system is different. We don't have results and clearing or anything. If you want to go to uni (or you'd like the option), you sit the Queensland Core Skills (QCS) test (this is the ONLY standardised test across all year 12 students - who want to go to uni - in Queensland). It's basically general knowledge. Your result has no direct affect on you personally at all, but your cohort's results are used to 'rank' your year level against others in QLD. Because course work/exams are marked internally (by your teachers, rather than exam boards), this is how they make sure no school is being marked unfairly high or low when it comes to their uni entrance scores. So if your cohort does really well, you'll get better overall position (OP) scores than a school whose students performed at a lower level overall. Your OP score is your university entrance score.
Okay so. Basically, the sooner you do a bunch of research, the better. If the move is likely to occur halfway through your A-Levels, you'd really benefit from putting in the groundwork now, to make sure your A level choices will put you in the best position possible when you move. To be totally honest though, it'd be much easier if you started Year 11 in Australia (the school year starts in January) and did the entire two year course there.
I hope all of that makes sense and is helpful. Seriously, feel free to ask me as many questions as you like on this.
Also! Australia is an amazing place. Whenever I've seen kids come over from the UK to start school here, the other kids are just super interested in them. I know moving countries is a big deal, but of any country to move to, Australia is a pretty good choice (although, I'm obviously super biased).