The only way you're likely to gain a university degree through mainly online study (and by postal tuition) is through the Open University:
http://www.open.ac.uk/
You'll still have to attend summer school but most of the work can be done from home. Remember, however, that once you've got your degree, the one-year post-graduate course for teaching can't be studied from home because you need to spend much of the time working in a classroom environment. (You can study for a PGCE part-time, over 2 years, but this still involves the same amount of classrooom experience).
Ther's more about information about qualifications and training here:
http://www.learndirect-advice.co.uk/helpwithyourcareer/j obprofiles/profiles/profile820/
Chris