The British are big optimisers. That's why the supermarket oligopoly (Tesco, Sainsbury's etc) have taken over many -- perhaps even the majority -- of small, neighbourhood shops (Tesco Express, Sainsbury's Local etc). The distribution centres and large vehicle fleet that delivers to the big supermarkets is reused to deliver to the neighbourhood shops, and many people then walk (or cycle) to those shops.
https://www.google.co.uk/maps/search/Tesco+OR+Sainsburys+in+...
https://www.google.co.uk/maps/search/Tesco+OR+Sainsburys+in+...
Milton Keynes is generally regarded as a boring, US-style city: https://www.google.co.uk/maps/search/Tesco+OR+Sainsburys+in+... (I've never been).
The British invented optimization in the WWII sense of the word, so yeah.
I don't know Britain beyond what I read of it, but I understand there's the concept of "little Britain" as a thing so maybe that's why small shops exist. In Montreal, I know there are actual bodegas in the city center and American-style convenience stores in the 'burbs.
No easy walking around town. All chain stores, very few independent shops. No hailable black cabs for some reason, everything involved calling a minicab and waiting. Just a terrible city to be in without a car and a designated driver.
Suburbs are unnatural and inorganic, and that's what people either hate or love about them.
This being said, there's frequently a "convenience store" that's sometimes closer than Big Mart. No veggies or actually even food, but they're there.