Dr. D is a political street artist who places his works guerrilla-style by taking over advertising boards and re-purposing them with his signs. I've seen his work in east London dozens of times, but I've never posted about the artist. Dr. D mocks the government, celebrities, and popular media. Below are two of the artist's common styles. The first is the "Curfew" sign, which mimics the vehicle congestion charge signs; if you weren't really paying attention, you'd miss the sign as they are so common. The curfew sign mentions "social cleansing".
The "H.M.P. London" (Her Majesty's Prison) sign is also a popular one. I've seen this before with additional accompanying text, but this one is placed on an abandoned building on Sclater Street. It seems to suggest that London is a prison. It works well on the building (which I wish I would have gotten a fuller photograph of) in its abandoned state with bars across the windows and doors.
These paste-ups are a great find because many people just walk by and accept the official-looking signage or advertising across the city. It is only when doing a double-look that you realise that it is a political message.