I left school a few years before the end of compulsory education, too, and will be starting TU100 in October, as well. I'm from the States, though, so it's not an entirely par relationship.
My maths education was very uneven. I was in an advanced maths class at one point, then when I passed the exams without coursework, I was put into basic maths. A couple of years later the teacher noticed that I was re-teaching his class after he finished every day, because they didn't understand his explanations, but mine made sense to them. He put me back into advanced maths, but the result was that I skipped trigonometry entirely.
I'd self-taught over the years from books, but always felt a self-conscious gap in my knowledge that was embarrassing. When it came to choosing between MU123 and MST124, it somehow came to a head.
My solution was to go over to khanacademy.com, and see how badly I'd done. It turns out, I'd taught myself fine, but every time I came to a trigonometric problem, I would re-devise formulae to get the information I needed, since I never learned it by rote or by practice. Khan Academy filled in these gaps very quickly and smoothly. It took about a week back in June, and I've since used the information at least a dozen times.