The best thing about any charity shop is that you never know what you are going to find. I have a well tested route around London that includes all of my favourite charity shops. This Oxfam has a great selection of books and often turns up some interesting clothes too.
Apart from the fact that its building is worth a visit by itself as it's the only purpose-built stable that functioned as a horse hospital within Central London that is still intact, it also offers some of the best underground and avant-garde culture in London. Aside from exhibitions, film screenings and music events it also houses 'The Contemporary Wardrobe Collection' which features vintage street fashion, couture items and accessories to the Film, TV and Fashion industries.
Beautiful architecture and setting, ideal place for catching the latest art-house flick or to enjoy evening tea and a journal session. Friendly staff, ambient atmosphere and a wonderful crowd.