An unassuming glass door on an otherwise plain facade hides a cavernous space, with high ceilings and huge sliding partitioned walls producing excellent exhibitions in a space you'll want to go back to.
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.