Excellent local charity The Bike Project is in search of new volunteers, particularly in the office which is close by in Herne Hill [thebikeproject.co.uk
]. Have been volunteering with them for a few years now and it's very satisfying and they are all absolutely lovely. Let me know if you want more information or get in touch with them directly through the website.
Also do think of them if you have bikes to get rid of or are in search of a bike to buy, their shop also sells various accessories.