Finish A levels first and then go abroad, I did the same.
If you would have taken Ad Maths, your life would have been A LOT easier(in maths) in A Levels.For all the time the rest of the guys would be doing P1/Pure Mathematics One, you could be out there bunking class(not that you should) and still know more than the guys sitting in class.
I am in college and doing calculus and I have yet to come across something which I didn't learn in Ad Maths and my semester is over in a week. That tells you about the advantage of Ad Maths.
Enough of the Ad Maths talk. Good luck with your exams.
When you are in A levels, make friends with teachers. Teacher recommendations go a long way in applying for college abroad.