That's very true, the math teachers often do know what they're doing.
And for maths... you need to practice hard. And have a solid foundation. Otherwise you won't make it.
Which uni, for curiosity sakes?
And for Maths, as I said, fill in the gaps of your knowledge and then spam past paper questions. If you run out, use MadAsMaths or other exam boards. Keep doing this until you can obtain at least an A, preferably A*.
Well maths is a flexible A-level subject.
And also you only have 4 months for god sakes. It's much better to stick with maths until the exams at least.