I would also suggest that you look at attending college/university 1st. 18 is not too young but once you have your licence its can be very hard to find your 1st Job.
A harsh reality about this career is that although you will have spent a small fortune, there are no gaurentees you'll get a job. If you look at the major employers you'll see that they offer sponsorship which may be one option in funding your training. These schemes are very competative and some even require their entrants to have a degree.
Its also a good idea to get more than your A levels as you never know what will happen in the future. You might not like your job or worse still you might loose your medical so a back up plan is always useful.
As mentioned above the main reason a lot of people enter at a somewhat older age is due to financing. Its a sad fact of life that we can't all have rich mummies and daddies to fund our training
.
In the meantime you can always do your PPL.