I think the best bet is to decide on what degree you'll want to study and go from there. I'm not sure how becoming a teacher works, will you have to do a degree then a post grad course or is there a teaching degree that gets you straight there? Find the degree you'd want to do and then email or phone a couple of universities and ask them.
It seems that a lot of universities want to see recent study at level 3. Depending on the course this could be A'levels, open university credits, access course or maybe even something you've done through work (NVQ/BTech). I'm guessing that to be a teacher there would be a minimum GCSE requirement too, you'll probably need at least English and Maths at C grade plus 3 others.
There isn't much funding for level 3 courses but GCSEs/A'levels can sometimes be taken at evening classes, open university is done in your own time. My access course takes place in the daytime, would your school be supportive? But like I said, speak to the universities first because you don't want to fork out money and time to find that what you are doing won't be accepted.