You don't have to have a natural talent at maths to do it at A level. It's one of the most commonly done A levels, and not everyone is a mathematical genius. As far as I know medicine offers tend to be A*AA-AAA, and with the assitance of "Examsolutions" and doing all the past papers I see no reason why you can't achieve an A. It was personally my favorite subject at A level because it's the easiest to revise, i.e the answer is either right or wrong. And TSR will answer any questions you have.
Hope I helped.