One of the more dispiriting experiences of the British supermarket is a visit to the fish counter. On a  historically seagoing island, the selection is often abysmal, frequently imported, and always expensive: farmed Norwegian salmon, farmed Vietnamese basa (blech), cod gone a suspicious taupe and priced like its weight in saffron (83 per cent of the cod consumed in the UK is also imported; why?) and maybe a few locally sourced mackerel or sardines, depending on the day.