Martin, welcome to Pprune Wannabes.
Your chances of getting into commercial aviation hinge on one rather important detail: could you get a Class 1 medical? You mention astigmatism, which may, if it is severe, prevent you getting this important qualification. I strongly suggest that, once you are in the RAF Regt., you see your SMO and get him or her to test you to the CAA's standards (actually, if you can get hold of the standards, you could probably do this at Specsavers!). It may be OK, it may not, but you need to know as early as possible.
I'm interested that you chose the Regiment as your way into the RAF. I'm not knocking it - my brother spent 17 excellent years in the Regiment - but it's about as far from aircraft as you can get in the RAF, which seems a little strange for one who wants to fly.
Your other concerns: academic qualifications are increasingly required of young ab-initio pilots but, as with most professions, the older you are the more life counts! At 26, you may find that A -levels and the like are less important than at 18. That said, it is always good to see a candidate who has made the effort to get extra qualifications after leaving school, particularly if school wasn't a great success.
As for finances, this is an expensive business to join. The least you can expect to spend in 2006 is around £40,000. Most spend a good deal more, and it looks like it may get more expensive still. At 26 you are likely to be on the old side for getting any kind sponsorship (though it's not impossible, and the market changes rapidly), so you must be prepared to find this cash yourself. The Regt. is not well known for offering an extravagant salary, so you will have to work hard to save the money. I'm not sure if the RAF still runs its flying club at RAF Brize Norton; if it does, and if you're based near there, you may be able to save some money at least up to PPL level by training with them when you can.
Good luck!
Scroggs