This milepost is, believe it or not, a listed building. But, it's not unique among mileposts in being listed, and sometimes they describe the listing as of a 'group of mileposts'.
Made of cast-iron, usually around 150 years ago, and now maintained by county highways teams, they are still much loved. Believe it or not.
Incidentally, I tried to find a name for its shape, which is a three-dimensional triangle with a slanted end-face. Such a shape has no exact name. The nearest that the Internet could come up with was 'Triangular Prism'.
This post was featured on the City Daily Photo's Theme Day pages