How the modern colour purple was invented in London

This summer, an awful lot of purple will be evident in London, from regal jubilees to the opening of the Elizabeth line, and not many people know that the modern colour purple was invented in London.
TfL publishes the Elizabeth line Design Idiom
Purple had been around for millennia, but it cost a literal King’s ransom to manufacture it, as a useable purple dye was only produced by a type of sea snail and it required thousands of snails to make a single cloak. So expensive was the colour th