If there is one dish that defines the trajectory of British food, then it is the prawn cocktail. Attributed to Fanny Cradock in the 1960s (perhaps lazily, as some say it has its genesis in the food preferences of a 19th Century Californian mine worker), prawn cocktail enjoyed a brief period as a beacon of high living, ‘going out’ and ‘having friends over.’ The key to the success of the…