The pillar box is a protected species.
Photo from Flickr user Redvers. Creative Commons licence.
George V pillar box
The first one was set up in 1852 on the island of Jersey, octagonal in shape and painted green. Red became the standard colour in 1874, although ten more years elapsed before every box in the UK had been repainted. At first each area could have its own design, until 1859 when a standardised version was adopted.
Apart from the stand-alone pillar box, there are also wall boxes:
Photo by Flickr user DavidT2006. Creative Commons licence.
Queen Victoria wall box
and lamp boxes. Lamp boxes were designed to be attached to lamp posts but often have their own post.
Photo by Flickr user Ingy the Wingy. Creative Commons licence.
Elizabeth II lamp box
They all bear the Royal cipher of the reigning monarch when manufactured. There are over 100,000 in the UK. There are even some in Ireland, where they were retained after independence, but painted green, and still bear the Royal cipher.
English Heritage agreed a joint policy (pdf file) with the Royal Mail in 2002 for the retention and conservation of these boxes. Some 198 of the oldest and rarest versions were already “listed” before this agreement was made.
This is a serious subject as presented by our mutual friend, Sheila, who really can't cope with the idea of anyone introducing Henry
into to any of her letterboxes. Too unseemly.