You need to remember that A-levels are only the first stage. You can't teach without a university degree, so (whatever route you take) it will be a long haul. (With full-time study, it usually takes 2 years to get A-levels plus a further 3 or 4 years to get a degree and teaching qualifications).
This link tells you about teacher training but it's really aimed at people who've already got A-levels:
http://www.tda.gov.uk/Recruit/becomingateacher /waysintoteaching/yourroute.aspx
Realistically, you'll probably either have to take a further full-time college course or attend evening classes.
If that sounds a bit too daunting, you might like to consider a career as a teaching assistant:
http://www.learndirect-advice.co.uk/helpwithyo urcareer/jobprofiles/profiles/profile180/
(As a teaching assistant, you'd get to work in the classroom without spending hours slaving over essays on comparative educational philosophies, developmental psychology, cognitive development and all of the other things that teachers have to study).
Chris