One of my favorite parts of British travel is that even something as basic as mailing a postcard can inspire a quest.
Postboxes first came into use during Queen Victoria’s reign and ever since each has been marked with the cypher of the reigning monarch. Initially there was no standard design so there are many different Victorian styles. Even colors varied before becoming a standard green in 1859 and then red in 1874. Over 6,000 of these Queen Victoria boxes are still in use.
Each subsequent design has the cypher of the King or Queen who reigned during its construction:
The most rare are from the short 320 day reign of Edward 8th:
There are already new Charles IIIs popping up:
This website has a searchable map to help in your quest: https://postboxmap.co.uk/. All of the ones I’ve posted were spotted in the wild, except the green Victoria box which is on display at The Postal Museum in London. The museum not only has a great variety of boxes including one of the originals installed in the Channel Islands, but also allows visitors to take a trip on the mail rail which from 1927-2003 carried London’s mail 6.5 miles underground from Paddington to Whitechapel.
Secrets of Philadelphia 