Ring your local college and ask to be put through to the exam department. I know that we have entered candidates for exams who haven't studied with us and charged an external exam fee. We used to enter people for almost any exam they wanted eg Dutch but I think now we only allow external entry for exams that would be running anyway. I think you should have no problem with A Level Maths - but you might have to do the sylabus that the college exam board provides.

All the best

Julie