Well, if you going to pay yourself through training then you don't need any. Although a basic understanding of maths and physics will help through your ATPL's.
If you are looking to be sponsored, then look to get at least two decent A levels as this seems to be the benchmark. In your case, get you head down at school, pass your GCSE's and A-levels. Three years is a long time in this industry so sponsorships (if they still exsit) will be probably a whole lot different to todays.
Alternately, you can look to join Betty Windsors flying club. Get to fly some very interesting types, but your subscription can last for well over 10 years.