I had a friend who resat maths with a B too, she did it because she was doing economics and wanted an A I suppose for when she was applying to university. It was no big deal, she sat it with a couple other people, got an A and that was that.
I don't think it really disrupted her AS Levels as it was a build on what she already knew rather than learning something completely new, so if you think you could do it, then I'd suggest to do it. But other than Durham, UCL, Cambridge and Oxford, I don't think many Universities look too closely at what you got a GCSE.