It's a real effect. Wikipedia has a whole section about it in their British cuisine article. A combination of rationing continuing well after the war ended (until 1954), agricultural issues, and a bunch of other stuff put Britain in a position where even after rationing ended, the effects stayed on. https://en.wikipedia.org/wiki/British_cuisine

It's not the only reason British food has a bad reputation, but it's part of it.

The real problem in my view is that people don't recognise post-imperial fusion cusines as 'legit' British food even though the curry house is the definition of an Anglo-Indian institution. When people in Britain claim the tikka masala it's not being twee or politically correct, rather it's earnestly considering an immigration-originated fusion cusine as part of the wider national one.

Also even pre-20th century British food isn't exactly bad, I'd argue it doesn't travel amazingly in many cases but a Sunday roast in our climate is sheer perfection. Too many people expect France or Italy from a country with nothing like their climate.