Or what A levels do I need to be a secondary school teacher?
Get into Teaching : https://getintoteaching.education.gov.uk/
Depends what you want to teach, you need a degree that contains 50% of the subject you want to teach. So if you wanted to teach physics, you would need an A-level in physics and maths to get onto a physical science degree. If you wanted to teach law then your choice of A-levels isn't crucial.