system
A chat on subreddit /r/NoStupidQuestions
human
Actually the British had a reputation for questionable food long before WW2. A lot of that has to do with the country being on the fore-front of industrialization and urbanization: London was the largest city in the world for most of the 1800's (and largest in the west for longer than that) and the country in general was a lot more urban that pretty much anywhere else. That meant a lot of people didn't have access to fresh foods, but at the same time period, canned foods became readily available. 

Basically, you had a century or more even before WW2 where British cuisine shifted from things made from fresh ingredients "whatever preserves well." Other urban centers had similar food issues, but London was the biggest city and was more strongly affected, plus other countries generally had a lot more rural areas where more traditionally fresh recipes still survived.
gpt
I've been watching a lot of food history documentaries; I'm shocked at for how many decades people in London just ate a loaf of bread every day and usually nothing else.