A vocational course (ie one with a recognised qualification) like an RHS course has little to do with gardening as such. It is unlikely to teach you how to grow, say, chillies.
I did an HNC a while back and thoroughly enjoyed it. It didn't teach me how to garden or particularly how to grow plants, but more on how/why plants grow, why certain pests/diseases are attracted to particular plants & how to get rid of them, the science of soil, nutrition (ie pH , why an acid lover hates lime), propagation, pruning, how to manage/organize a garden, horticulture vs. agriculture, and of course the obligatory plant identification.
Its certainly an eye opener to the world of horticulture, rather than the limited life of a gardener.
A formal course is an academic exercise, not a practical one.
It may not have taught me directly how to grow things, but now I have a much better understanding of what I need to do to make things grow and why. It has removed a lot of the trial and error guesswork I used to have and has given me the confidence/ability to talk intelligently about the subject.
Sparkly, I suggest you get a copy of the syllabus, and see if its the kind of thing your looking for.