There are a fair few correspondance colleeges who do a preparatory course for the RHS levels 2 and 3. They dont put you in for the exams though.
I paid for the 2 level 3 courses with the Horticulture Correspondence college but having looked at the sylabus and past papers on the rhs website you could probably do it on your own if you had access to some good books and internet.
Have a rumage around the RHS courses pages and see if its something you would fancy doing. http://www.rhs.org.uk/Courses/Qualifications
and they have a list of colleges to so you might be lucky and find one near you after all.
I'd have loved to have done it in a class of other keen gardeners rather than at home by myself scrathing my head over plant hormones!
Its interesting stuff and can give you some great practical insights but its heavy on the biology.
Give it a whirl you might love it!