Good idea to apply yourself towards obtaining good grades at GSCE and A-level. The BA FPP scheme sets a particularly high benchmark in this regard, and obviously a good solid set of education certificates is only going to be a good thing generally.
Your presumption about airlines only taking on pilots with 1500 hours isn't accurate, however the routes into "cadet programmes" are very competitive and often very expensive. There is a lot of advice, comment and discussion on these boards. I would recommend taking the time to read through those that seem relevant to you (particularly some of the longer threads.) You will find many of your questions have been answered time and time again. Once you have a better understanding generally, post on the many specific subject questions that you will no doubt have.