Like others have said, the war ‘locked in’ perceptions but there were years of supply shortages and government rationing of foodstuffs afterwards, ending in the early 50s.

Cookbooks were re-written at the time to emphasize economic meals like stews and soups that used a lot of vegetables you can grow in the garden and to use up scraps. Potatoes, leeks, and cabbage grew to feature much more heavily, meat was OUT as was sugar and most fruit… You try working varied and fun meals with those limits

So combined with the more ‘bland’ cuisine of the era and extensive rationing limitations, generations of households got used to cooking and eating the basic foods. Immigration and globalization has broadened the modern offerings, but the British classics’ are undeniably under spiced shades of grey and brown.