I think it's because many British people culturally don't value food much. Supermarkets are particularly poor. You need to go out of your way to get nice things.

It may not be as bad as it was in the 90s, but I still find nicer stuff in supermarkets in Spain, France, Germany.

Of course I'm not usually shopping at the equivalent of Tescos on the outskirts of town when I visit a supermarket on the continent, but I'm not exactly searching hard either.

Food in London is great, and varied. I don't much enjoy most Asian food, but London has a lot of it, in a way that e.g. Paris isn't as good at. OTOH I've had a much better hit rate, and at cheaper prices, in Barcelona for great food in restaurants, ethnicity aside.