In the image we can see an example of the current mailboxes in the United Kingdom, in which the royal monogram of the Queen isabel II (mailbox on the right), and another example of the mailboxes still bearing the royal monogram of King George V (mailbox on the left). You have all the information in: «How does the postal service work in the UK?«.
You have all the information in: "How does the postal service work in the UK?".