Fish and chips became a stock meal among the working classes in Great Britain as a consequence of the rapid development of trawl fishing in the North Sea, and development of railways connecting ports to cities during the second half of the 19th century, says Wikipedia.
In chip shops in the United Kingdom, salt and vinegar is traditionally sprinkled over fish and chips at the time it is served.[22] Suppliers may use malt vinegar, onion vinegar (used for pickling onions), or the much cheaper non-brewed condiment. A portion of mushy peas is a popular side dish.