I asked an ex work colleague, a Jerseyman who lives there, about the spuds. He says "They do say that it is the use of artificial fertilizer instead of traditional seaweed."
Found this also:
Quote They do say that it is the use of artificial fertilizer instead of traditional seaweed.Each bag of Jersey Royals sold can be traced back to the farm on which the potatoes were grown, guaranteeing taste and quality. The name Jersey Royals is emblematic of the sun soaked côtils, the fresh sea breezes and, in some cases, the ‘Vraic’, or seaweed, which is used by farmers as a natural fertiliser.'"
Maybe somebody, probably not a supermarket, is importing the real thing?