I was sort of in the same position as you. They are both extremely difficult fields to get into. Law is very competitive and there's an excess of Law graduates with many being unable to secure training contracts and end up in other jobs. However, if you work hard now and aim to apply to top unis and end up with a 2:1, you'll be in a better position than other graduates.
I'm sure I don't need to tell you how hard it is to get into Medicine, but if it what you are truly passionate about and want to do then don't let that put you off.
I'd advise you to keep your a level choices somewhat open so both doors aren't closed. Maybe do something like Chem,Maths, Bio and History. Also try to get work experience in both fields.
Apply to access programmes and summer schools too.
Some of the things I've done to enhance my law PS:
-part of the Pathways to Law programme(I strongly advice you apply for this)
-2 week work experience at DLA Piper
-summer schools at KCL and Oxford this summer
I don't know any specific access programmes for Medicine but I'm sure you can find some.