Hi There,
I am one year ahead of you in the education system. So first year of A-Level. I would also like to be a pilot. I think a backup option is essential. So you must think about that. Say for example if you fail your medical for whatever reason.
I am taking Maths, Physics, Geography and Spanish at A Level. You should take Maths and Physics and the other 2 are up to you really. Whatever you enjoy.
Regarding the training. I would look at the two sponsors. Whilst the competition is incredibly intense, whats the worst that can happen if you submit an application.
Cathay and Etihad offer training schemes fully endorsed. I would look on the airlines websites. Plently of info on both schemes is available here on pprune.
I would also think going abroad is a good option. As far as I can see the only way to get into the industry here is to fork out a dirty great sum to the airline in orange or their irish competitor. This doesn't even guarantee that you are in. I've heard stories here on pprune of people being on a temp contract and sent packing after 6 months.
You must also remeber all the flight schools that you see advertising like FTE, PTC etc.. are ultimatly a BUSINESS and have a very good team of persuasive salesmen, so be careful.
Perhaps wash planes at the local airfield in exchange for some hours. Mingle with people at the airfield. (Contacts in aviation go very far). Whenever you are on a flight ask if you can visit the flightdeck (I have never been turned away) They offer very friendly, impartial advice which is very helpful.
Good luck!