First of all, if you only managed CD in GCSE Science, maybe you should rethink your choice of A-Levels, because they are really hard, especially the Science's and even people who get A's can struggle. Medicine is a really competitive field and the very minimum they will expect you to have in Science GCSE is 2B's.
But if your set on a career in the medical field, then if you know the exam board, see if you can find their list of exam centres for external candidates. I think google should come up with it. And you should really retake both Science GCSE's, because unfortunatley for something like medicine a C really wont do you any favours.
Some Unis also ask for a minimum of 6 B's incl. English, Maths & 2 Sciences. So you would be limited as to where you could do medicine or similar, as its one of the few courses to have specific GCSE requirements.
Hope this helps!