It's not a stupid idea at all and will enhance your BTEC qualification either for employment or uni study.
1. Obviously I have no idea what you are capable of but many people self study AS and A level maths and do very well. What's your GCSE grade? That would give some indication of your likely success. Try it with just text books and web resources, ask for help in the maths forum. I would recommend spending spare cash on a tutor rather than a distance learning package.
2. It's very possible your college would let you sit as a private candidate, talk to the exams office. You'd have to pay the fees but it would be your cheapest and easiest option. Note, you have missed the deadline for normal exam entries for this summer which makes everywhere less likely to take you for this year and the cost would be much higher. If your college won't do it, check with other local colleges and independent schools to see who takes private candidates. All of the examiners have advice for private candidates on their web sites.
3. Find out which examiner you will be taking and buy text books that match the examiner. Come back and ask for recommendations when you know the examiner.
Loads of great free resources on the internet including
www.examsolutions.net and
www.m4ths.comPosted from TSR Mobile