Barnes Cross Postbox
The pillar box outside Barnes Cross Cottage in Holwell near Sherborne is claimed by many to be the oldest box still in use in mainland Britain.
It was made by John Butt & Co. of Gloucester, to an unusual octagonal design commissioned by the Surveyor of the Western Division of the Post Office, Anthony Trollope, in 1853.
These were manufactured in batches between 1853 and 1859
Wikipedia reckons the record actually belongs to a pair of boxes of a slightly different design in Framlingham, Suffolk, which were manufactured in 1856, and that the Barnes Cross example came from a batch produced after this date. But then Wikipedia has been known to be wrong.
Everyone seems to agree though that the oldest box still in use in the whole of the British Isles is in Saint Peter Port, Guernsey and also dates from 1853.