You'd need to buy seed potatoes called 'International Kidney'. These are actually what Jersey Royals are. The difference in flavour is because, traditionally, seaweed is used as the fertiliser in Jersey. In Jersey they are lifted as 'early potatoes' while they are still immature. If left in the ground, they will grow much bigger, and they end up losing their flavour. In the mainland UK, planting is usually done between February and the end of April, depending on local climatic conditions.