First, what would be the point of taking the exam again if, as you stated, your "maths skills are primary school level"? The websites I mentioned can be used to teach yourself mathematics and thereby improve your chances of obtaining at least a C grade.
Second, some jobs do not require GCSE maths. These are usually crappy jobs, but if you save some of the money you earn whilst working, you can use that money to pay to take the exam again. If you teach yourself mathematics, then you would only have to pay for the exam, and not an entire course.
Finally, a lot of employers do not check your exam grades, especially if you have a university degree and/or you are applying for a low level job, so you can always lie on your application and claim that you achieved a grade of C on GCSE maths. If you do not have the required skills, and this becomes evident whilst you are working, then you are in trouble. Again, the websites I mentioned can be used to improve your skills.