I have to disagree with you there. You described perfectly the difficulty in getting a place in medicine which makes me wonder why you said this. I'd say that if you want a serious chance of getting in, deciding post-GCSE results is the best option. That way you have about a year and a half before you apply. This is the perfect amount of time to decide you want to practice medicine and as a result, prepare for the application process (i.e. experience/volunteer/extra-curriculars). Also, unlike other courses (though similarly to Oxbridge candidates I think), medical applications need to be in by mid-October. I don't believe that a month and a half is enough time to seriously decide you want to do medicine.
I'm not saying it would make things impossible, it would just make them damned difficult.