Not sure about London as I don't go often at all. Fish 'n' Chips is like the British go-to takeout haha. It's great to have walking along the beach lol.
I live in a seaside town in the South-West of the country. A lot of the "local" restaurants I go to, although they offer different menus etc, a lot of them offer burgers. It's actually hard not to find a place that doesn't do burgers when you visit a local countryside restaurant. My mates often laugh at me when we go to eat out at a restaurant as I
always choose a burger.