Dave,
you've heard all this in various ways, but let's see if I can present the argument in a way that makes sense to you.
At 17 you have one great advantage over all other wannabes, and that is time. Your challenge is to use that time to add value to yourself so that when you come to look for a job (in whatever field) you stand out above the average applicant. That is easily explained, but bloody hard to do.
You have decided to leave your A-level course? Well, that puts you at a serious dieadvantage against all those who continue. You don't need A-levels for a flying job, some will say. No, you don't. There are some even in BA, Virgin, etc who don't have them, but not many - and almost none of them are under 40 years old. You could well, after 5 or 6 years hard graft, end up with an air taxi job without those A-levels, having a lovely time in your twin piston, visiting the great airports where those better qualified (and, by now, younger) than you are just getting into their Airbusses and Boeings and jetting off to parts of the world you'll never see - unless you pay for it yourself.
Of course, you might say the world is full of those who have succeeded without academic exams. Well, acyually it isn't. There are a very few, highly visible, entrepeneurs that have made it (and, even without the exams, they worked their arses off to get where they are). For every one of those there are thousands of more ordinary folk who took the conventional route. Airlines do not employ unconventional people, believe me.
So, how can you use your time to get an advantage over the other Joe Bloggses out there? Go to watever educational establishment you can and get those A-levels as quickly and as well as you can. Then get your Class 1 medical and do the GAPAN aptitude tests. If you're still considered worthy of flight training, get on a PPL course and start applying for sponsorships.
To reinforce the point, the rarest pilot in the airlines now is the under-30 chap with no A-levels and no financial cushion to get him through unspomsored training. Why make it difficult for yourself?
To put it another way, why should anyone help a guy who refuses to help himself?