This is a Royal Mail Box, known as a Bag Drop Box, and it is used for the sole purpose of storing letters and general mail for a postman/woman to top up their bag whilst on their round. This way they can ease the load by delivering the mail in batches, rather than lugging it all around in one back breaking go!
The cache is a magnetic nano, so you may need tweezers, and you will certainly need to bring your own pen.
Thanks again to 'Tonymusprime' for giving me the nod to place a few of these.