In one of his pre-concert talks a few years ago, the conductor of the Hartford Symphony mentioned taking "conducting classes" (I forget the exact phrase, unfortunately, thinking that nobody would ever ask a question like this where I could see it!) with a professor at the Yale School of Music. It might have been "conducting lessons" or a "conducting seminar," but you get the idea.
Being American, I'm not sure what A-levels are, but they sound important enough that somebody at your school should be able to put you on the right track. You could also contact Sir Colin Davis, or anybody from a regional orchestra, and just ask. I mean, you've got to have a lot of self-confidence to be a conductor, so you might as well start out by getting in people's way and asking for help
Go for it. If anybody pisses on your fire just for having ambitions, it says something about them and nothing about you.