A level as entry requirement to PGCE program
In the UK it should definitely be possible for you to become a maths teacher with your background, but I'm not completely sure whether you would need to do A-level maths first.  I would expect that most maths teachers would actually have a degree in maths or a very closely related subject, but I don't know this for sure.

You might find helpful information on the UCAS website (the link is to the page about entry requirements for teaching).

From that page:


  Obviously, you'll also need a strong understanding of the subject you
  want to teach before you start training. If your degree subject
  doesn't link closely to your chosen teaching subject, we'd advise you
  to take a subject knowledge enhancement (SKE) course before starting
  teacher training. You can only do these in England and in certain
  subjects.