The postal service is forced to change

The First World War was a major turning point in the history of the Post Office. Many of the services that were reduced because of the war were never the same again.
The volume of mail increased dramatically between 1914 and 1918, rising from 700,000 items in October 1914 to 13 million, but the number of Post Office staff diminished as men left to fight. This meant that the Post Office could not continue to offer the services it had before the war.
In 1914, some areas in Britain had post delivered up to 12 times a day. During the war extra work and a reduced workforce in London meant that deliveries at home were cut: London was reduced to six and rural areas four or even just one. The number of Travelling Post Offices carrying mail around the country on the railways was also reduced.
Since the first stamp was introduced in 1840, it had cost one penny to send a standard letter. The cost of the war however led the government to consider an increase in the postage rate to help raise funds. In 1915, the maximum weight for a standard letter was reduced to 1 ounce (28 grams) and the cost of sending it was raised to 1½ pence in June 1918. The cost of postage never returned to one penny thus ending the universal penny postage principal.

