Morley College (SE1) courses are good - they have some fab one-day taster things, usually at weekend. The longer courses are good too, but it can be a hassle to get onto at beginners level, especially for the Autumn term.
I definitely improved technically as a result. Although in teaching me to be a bit less rough'n'ready, I'm probably a bit less inclined to be inspired just go for it, than I used to be; which is a shame. Although I make fewer, hem-hem, unique fashion disasters pieces
It's not really central but I'm doing some day courses at Oh Sew Brixton and they are fab! I've done the beginning day where we made a bag and learnt cutting out patterns, embroidery stitching etc. Then alterations, seams and hems etc. Next up is te make a skirt day and then wardrobe cloning course, where you take a skirt or dress you love and they show you how to copy it in different fabrics. There is a six week dressmaking course that I'd love to do but can't commit to, and cushion making and jersey dress making that look interesting.