I disagree that any decline started in the 1970s. Quite the reverse. It started to recover then.
The biggest disaster by far was the rationing in WW2. A whole generation grew up with no food to learn to cook with. My late mother was a) French and b) a terrible cook. Her family moved to Britain as refugees in 1939 when she was 10. Rationing began the next year and lasted until 1954, by which time she was 24 and a mother (to me and to my younger brother). As a child she was not allowed to cook as, if she screwed up an ingredient or a dish, the family went hungry. You couldn't nip out and buy more food. My grandmother cooked. The same applied to most families.
I remember in the 1950s, the paucity of available foods. No garlic, the lack of some of the most popular fruits, minimal choice of vegetables etc. Olive oil only being available in tiny bottles from a pharmacy to be used to treat ear wax blockages!
In the 1960s and 70s, the advent of cheap travel (mostly to Europe) led people to discovering or rediscovering better, more interesting foods and becoming more demanding. People learned again the food could be good!
This coincided with the rise of supermarkets, offering an increasing number of choices.
Sure, so called "convenience foods" but they are not particularly British. They are mostly American in origin but universal. As is Findus, although headquartered in the UK.
I discussed all of this (and some others such as Full Breakfasts and Borough Market in this topic.
