I think you should either just continue with A2 and get the best grades possible (ideally at least AAA for medicine) and then apply the year after (so take a gap year doing volunteering etc. build up your personal statement) with your concrete grades. I don't think you should retake the year as most medical schools will not accept your application (though you never know if you don't call and ask, though they will expect mitigating circumstances). Only retake the year if you really think you won't make the needed A2 grades because good grades are better than bad grades, and with the linear system it might be difficult in terms of retaking year 13 exams. I know someone that had to teach herself for a year because her school said she'd have to do the whole two years again