The majority of answers on here are leaning towards the 'join straight away' ethos, a point of view which I respect and understand. Here's my view;
I was in the exact same shoes as you five years ago, I left school, got virtually all A's at GCSE and the world was my oyster (kind of!) Basically, I applied for the Royal Marines, and applied to do A Levels at college but kept my forces applications open, so as to keep all avenues open until the latest date. I began to like college and decided to stay, without attempting my PRMC etc. I'm glad I did this, because now, I have 4 A' Levels, and I also have a degree, which is allowing me to join as an Officer. As Grey Wing rightly says, my qualifications will be old A' levels in the future, however, A' Levels are not simply skill courses, they are Academic courses, which aim not only to educate, but to improve general academic capabilities, something which will never go out of date, and will always be useful. Going to college and Uni I have learnt a lot about myself and where I want to go. I know I want to be a Marine Officer, and I have no doubt whatsoever in my mind. Studying has also given me the tools to research my interest in the Marines further, and gain a greater understanding of what the job entails etc. All along, strengthening my application.
My advice to you would be this;
1st) Study as far as you possibly can whilst its free and convenient, imagine leaving the marines at 35, been on tour around the world, lived the high life etc etc, and then having to go and study. Nightmare.
2nd) The marines arent going to go anywhere, college gets further and further away the older you get, the more responsibilities you have.
3rd) Scenario; You join the marines this year, pass PRMC very well, begin training, get half way through training, and break your back (touch wood). What are you left with then?
4th) If you are questioning joining, then you really need to re-consider. It's not something you should do if there is any doubt in your mind at all. I had doubt, didnt join, studied, now i'm joining, and have absolutely no doubt in my mind, at all that this is what I want.
5th) I dont care what anybody says (no disrespect greywing) but A levels will never be irrellevent. How much can human biology change in the next 30 years, 100 years, 1000 years, my guess is not a lot. How much is Maths going to change? English? Music? Physics? Not very much at all. Not only do you learn the subjects you study at college, but you learn how to learn. Properly. Something which no other course can teach you. I can understand how GW has got this idea that an A Level in technology will be irellevent in 20 years time, but I think maybe he's getting mixed up with GNVQ's, diploma's etc. Which are generally aimed at teaching skills i.e. VCR fixing. Hence the title including 'vocational'. What you learn at A level will never leave you. If you are considering GNVQ's, then I would tend to agree with GW. What you learn in them courses is useless unless practised consitantly on copletition of the course.
Sorry to rant, but I think it's very important that young lads like you consider everything very carefully.
Lee