I watched the program "Imagine" last week and if you get a chance its a great watch about probably the very first proper street photographer. His imagery is impressive but also the story goes to tell how he brought some of that candid work into editorial work too. http://www.bbc.co.uk/programmes/p011gc0d
Yes Alison its at the Tate Modern on the Bankside and I think the specific exhibition runs through to January at about £12 admission (usually Tate Modern is free entry except for exhibitions like this) http://www.tate.org.uk/visit/tate-modern