He proposed British readers to interest themselves in the arrival of the euro currency in many countries since 1999: double denominated stamps, both currency on covers, etc. Demonetisations of stamps in the older currencies too (but France, no general demonetisation since 1849). But it concerned the United Kingdom too.
(Effigy by Edmund Dulac, illustration and design by Eric Gill)Three demonetisations took place on the other side of the Channel (for once, the French expression is shorter: Outre-Manche) and in Ireland for the first one.
In 1915, the stamps picturing Queen Victoria, who died in 1901, were no more usable on mail.
In 1930, it was the turn of King Edward VII stamps, deceased 1910. The process was slow and not too shocking for collectors proud that their stuck-in-album stamps were still valuable for franking.
The adoption of the decimal monetary system in 1971 rendered obselete, on 1st March 1972, the stamps in shillings (s. or /) and pence (d.). The one of one pound (no more) were kept. They are from the Castle and the Machin series.
Let's add to these the possibility to use on United Kingdom mail the British stamps sent to the British post offices abroad, on the condition that the overprint did not modify their face value. How many collectors played to stick Nauruan Georgian stamps on their mail.