I can see why some people think it would be a good idea, as I can imagine that there are many parts of a medicine degree not covered by A-level biology or chemistry. But there would be loads of logistical problems with setting it up. Who would decide what's on the syllabus? What if someone took it and then decided against medicine? If it was a slightly 'vocational' A-level, would universities accept it for other academic subjects?
And of course, private/grammar schools with plenty of medicine applicants would offer it, while comprehensive schools or schools with less medicine applicants would most likely not offer it - meaning that those pupils would be at a further disadvantage.