Depends where you live. They're not in many parts of London.
Also depends what you're looking at. Local supermarkets sell a very limited and very expensive range of fresh veg. Again, depends where you live, but in many parts of London there are local shops that sell veg that are much cheaper. Where they are cheaper, they can often survive, admittedly.
With larger supermarkets, they can kill shopping streets by driving the local butcher, baker, etc out of business and having them come and work for the supermarket. Short-term, people may prefer to shop at the supermarket. But I would use fresh meat as the example of what is now happening in supermarkets - they have eliminated the competition and their prices are now really very high. Fresh veg likewise.