animal shelters, food donation centers, children's mentor programs (like Big Brothers/Big Sisters, I'm not sure of organizations in the UK)
My boyfriend did some volunteer work for his company (computer help desk), and they worked on computers for a non-profit organization. So sometimes there's stuff you wouldn't even think of.
There's a section for volunteer work in the classifieds in some of the papers I read. You'll probably have better luck with alternative newspapers, like Creative Loafing. Also check out Craigslist.