I was peeking at the London page on Flickr, and I noticed “cansfestival” in the hot tags. Turns out The Cans Festival is a stencil art show happening underneath the Waterloo tube station. There’s some lovely stuff from Banksy, amongst about 40 other artists. There’s more information about the show in this Guardian blog post, “Looking for radical art? Try the South Bank, not Banksy.”
Thanks to ((morado)), Herschell Hershey, _Kriebel_ & MykReeve for their documentation of the event.