Remember you need an A in whatever third A level you choose. That third A level doesn't have to be maths. If there is something you enjoy more and feel like you're stronger at then choose that subject over maths.
In my opinion, if you think you're only gonna scrap a B in GCSE maths then you will probably struggle with A level maths (you would have to put a LOT of work into it). I'm not really informed about use of maths but I have heard it is seen as quite a soft subject, this may disadvantage your medicine application. However, to be sure it is best to email the medical admissions of some different universities as to whether they will accept use of maths or not.
I have an offer for medicine right now and although I do biology, chemistry and maths I know a lot of offer holders whose third subject is different (e.g. economics, psychology, French, etc.) I think that it is important to choose something you're good at and you enjoy because otherwise you will probably struggle.
Good luck with it all. If you have any questions feel free to PM me (might not reply till Friday since I've got exams this week).
Lauren xx