Maths at A level is mostly about work, you will literally get out what you've put in. Natural aptitude doesn't really come into it, what does is how much you want to do it and how much you enjoy it. If you're only getting an B at GCSE, you should first work out why. Chances are you just haven't worked enough at it, and that's because you don't enjoy it enough to get motivated or enjoy learning the material. If this is gonna carry at A level, then you should think twice, because you don't want to be fighting against something you don't enjoy for 2 years at a fairly high level. If you're motivated however, then there shouldn't be a problem