If you can prove to your teacher that you'll work extremely hard to get high grades at A-level, they might have you as an exception if you were to get a C grade (also it will help if you're in their good books!)
A grade B is a very good grade to have in Maths because it proves you have an ability to do Maths which is above the national average. Generally, if you want to go onto university and study a legitimate course such as competitive Scienes/Maths/Business Studies related subjects, they will require a B or higher in GCSE Maths. So it's not a bad idea to try and get a B, but don't fret - getting a grade below the required GCSE grade isn't the end of the world - they're only a guideline and you can definitely still be accepted if your personal statement and teacher's reference are good. And of course, your A-level results are much more relevant than GCSE results at university. :)
Don't worry about it, try and block it from your mind until exam results day - what's done is done. And, chances are, if you found it a difficult paper, so did the rest of the country. One year for one of my papers, an A grade was as low as 60% or something. You could very well get a grade B so do not worry!