I've been thinking about this quite a lot. I think things are far from lost for you sainty.
If you were to go and get your GCSE's and A levels by the time you are 23 you will have a couple of problems. Firstly, is that you'll only have one shot at OASC or AIB, so you'll have to get it right first time. Secondly, everyone else who is 23 and applying will most likely have a degree of some sort.
As such, you'll need to get your GCSE's and A levels to high standards (I'd aim for at least B's in everything). If you can do this I think you will show good character and this will reflect well upon you at OASC. At the end of they day, you'll just tell them that things didn't go well first time round, and that this made you realise just how much you wanted to be a pilot. You've then gone back to school or college and done what is necessary to get it. To me that would show great determination, motivation and commitment to the RAF. Coupled with the fact that you've got a PPL this would make you a reasonable character on paper.
Obviously, that is only one part of the selection process, and sucess is far from guaranteed. I had to go through OASC twice before getting selected for my bursary, and I can tell you that there is a lot more to the selection process than pure academics. You may not have a Masters in Engineering, but you may still make a good Officer.
All the best.
Noos