Why do you think its going to be a huge workload if you don't start now?
Kquishir, don't worry, if you want to start Maths now, by all means go ahead. If you think it will help you later on and save time but sometimes it is better to learn under the supervision of teachers (my opinion anyway). Personally I would say just enjoy your holidays! You've done so many exams and everyone can use a break!
If you want to then do it casually, don't over work yourself...
It's your holidays - use them for whatever you want. Maybe do a little, but don't be afraid to abandon it fo a while to go out with friends/go on holiday and so on...
If you take additional mathematics at GCSE level (and understand it reasonably well), AS Level maths should not be much of a problem.
Thats what I heard.
Ye C1 is mostly GCSE stuff to be honest:
Powers (eg. x^m/n = nth root x^m)
Solving Quadratics (the formula, completing the square)
Geometry (quite easy - calculating gradients and using y=mx+c in different ways)
Sequences (one of the harder parts - but you don't have to remember the formulae)
Calculus (once you understand it you can do it in your sleep)
I haven't done C2 but it contains trigonometry and exponentials/logarithms that aren't covered at all in C1.
Knowledge is power. If you're going to start maths then do it as it's a blessing and increases your enthusiasm for the subject.