Geograph - photograph every grid square

A blue plaque is a permanent sign installed in a public place to commemorate a link between that location and a famous person or event, serving as a historical marker.

The world's first blue plaques were erected in London, England in the nineteenth century to mark the homes and workplaces of famous people.
The original blue plaque scheme started in the 1860s, is now run by English Heritage in London. It is believed to be the oldest such scheme in the world.
There are currently about 850 plaques in London.