Depending on how often you plan on flying to gain your PPL(H), you can start now if you want but you cannot fly solo until you are 16 and you cannot have your licence issued until you are 17.
Whilst it is reasonably common for PPL(A) students to complete their licence in the minimum hours of 45, it is less common for PPL(H) students so budget for 50 plus hours. If you get it in less that's a bonus. In addition, it is less likely that you will fly solo early on in your course; maybe after 10-30 hours depending on your aptitude and that attitude of your instructor. Flying an hour ot two a week should give you some sort of timescale. I am assuming that you are thinking of a modular course rather than integrated.
Go and visit a couple of local helicopter schools and have a chat. Look at the operation and how helpful and professional they are. There is also the matter of a few little exams for PPL; you might want to start studying for those straight away (Books - Trevor Thom or Jeremy Pratt). Most schools will require that you have passed Air Law before first solo.
It's hard to advise about A Levels or apprenticeship but, if the apprenticeship will give you a job at the end (if you pass) and a vocational qualification recognised in the industry, that might be your better bet.
Hope that helps.
Cheers
Whirls