cod or haddock
Joseph Malin opened the first recorded combined fish-and-chip shop in London in 1860
The high fat content of the batter that they are cooked in and lots of calories and carbohydrates in the potatoes.
Normaly a fresh piece of fish and some sliced potatoes fried in oil.
I believe that Londoners prefer cod with their chips. More civilized Northerners prefer the infinitely more delicate and flavoursome haddock. The Scots are given to frying Mars Bars or so it is said.