It’s funny that it should be Americans who get so hung up on the awfulness of British food, because late-20th-century American food was similary appalling—my father is still traumatized by the cuisine he encountered as a university student in the States in the 1960s.
There’s been a long-term contretemps in the U.S. between those who live to eat and those who eat to live. We settled that by creating an industrial feeding system so that we eat to die young.
There’s a basic truth here: a lot of—especially working-class—British food from the 20C is awful. This is similarly true of American food. But both have changed dramatically since the 1990s, and while we recognize this of American food we have a weird reluctance to do the same for British food.