OK, so I'm going to cheat slightly ...
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... but only slightly, as this is a special pillar box. It's an early Victorian model and there aren't many of them still around. This one is in Eton High Street and must date from around 1855-1860.
The pillar box (and the wall-mounted post box) is something we tend to ignore; they're common and we use them regularly. Yet they are an enduring piece of British life as well as being a very good and functional piece of design. It is surprising how old some of them are, but then they are mostly made of highly durable cast iron and are well painted. It is also interesting how ornate some of the Victorian pillar boxes are: the hexagonal ones (which are more common than this "Greek column" design) are especially good, their top being in the shape of a (flattened) crown. Some, like this one, are actually listed buildings!
You can always get a first guess at the age of any pillar box because every one carries the insignia of the monarch at the time it was erected. On this one you can just see the end of the VR, for Queen Victoria, at the top left. Notice too the very small vertical aperture.
The pillar box, although originally suggested by Rowland Hill (he of the Penny Post), was actually introduced by Anthony Trollope (yes, the novelist) whose day job from 1841 to 1867 was as a Post Office Surveyor (first in Ireland and, from 1851, in Eastern England); he lived for many years in my home town (Waltham Cross). The early boxes were of various colours, with green being the initial standard (there are still a few green ones around; there's one in Rochester, Kent) with red being adopted from around 1874.
There's more on the the history of the Pillar Box on Wikipedia. An everyday object with some fascinating history.