Discoveries like this just go to show how sophisticated the Roman trade network was, and how devastating it was when it all collapsed. Purple then basically disappeared from Britain (it was extremely rare) for a thousand years until the 1400s when trade started expanding again. That is the basic story of a lot of these finds. Something common or at least more common in the far-flung corners of the Empire in Roman times disappeared for a thousand years after the Empire’s collapse.
Imagine how devastating it will be once America collapses and the world can no longer access a Happy Meal with fries including a Mayor MacCheese action figure.