Found than to Machin Mania, this video by Chris Phillips was posted last year on YouTube. Subject: the evolution of the simple letter rate in the United Kingdom from 1840 to 2008.
The first part permits to retrieve the British first types, whose surnames are so exotic when a French philatelist find them in a magazine article.
The second part became a color study of the Machin series: 40 years of use during the Western industrial crisis of the late 20th century.
Only a note on my part: about the calculation. Yes, 2008 rate = 86.5 times 1840 rate... but this multiplicator is no information if you do not know if you need 86.5 times salary work to send a letter today compared to the mid 19th.