From what I’ve gathered, it can be both the “fast food” of London (although can you imagine McDonald’s selling large battered fish? At the moment, McDonald’s got nothing on fish n' chips), and it can also be a fancy meal. Because it is a large fish, it’s kind of expensive, usually around seven pounds (about $10).
It's a popular dish because London is right next to a river, and there is access to a lot of fish. Sort of like how Washington State has great-tasting apples because we have so many apple orchards near Yakima and Wenatchee.
