Not sure what benefit you could possibly get from a medicine A level? What areas would it cover? Physiology, pharmacology, pathology, histology, biochemistry, immunology, anatomy, sociology, psychology, embryology, and that's just the preclinical areas that have come off the top of my head. My favourite thing about studying medicine is that it is a hodge-podge of loads of subjects all drawn together, but an A level in medicine would presumably just be synopses of these disciplines; there is no way that you would gain anything. The only way to teach it would be to dumb it down, so when you find yourself sitting in your first undergrad medicine lectures a few years later, they'd have to re-teach it all anyway.
An A level in anatomy would be awesome, though, and I would have taken it in a flash.