Hi there,
I am in the same boat as yourself starting AS levels this September and hoping to join an integrated course thereafter.
While you are doing the GSCE's I think the most sound advice is to work hard with them, get good results and also build on your extra-curricular activities. For example join the ATC (Air Training Corps) if you're not already in it. There is no obligation to join the RAF but you can learn an incredible amount of experience there. You can fly/glide, and build on your social skills. You can also do the Duke of Edinburgh Award and this also shows team skills.
Regarding the cost well this varies greatly. If you were to go modular then the cost is around £40k whilst if you went integrated the total comes to around £70k. These totals are excluding a 'type rating' which some people buy whilst others may get theirs paid for by and airline and be bonded to them as a result i.e they have to work for that airline for say 3 years at a certain salary. A type rating allows you to fly a certain aircraft eg Boeing 737. This costs around £20k.
If you could let me know what abbreviations you require i will do my best to let you know what they mean.
All the best
Arpan
PS: Check your PM's.