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.
Sainsbury and Tesco are on par with Carrefour and Mercadona. Waitrose is on par with or better than Albert Heijn. But the Chinese supermarkets through Europe have the best value and range of ingredients for cooking, imo.
>I don't much enjoy most Asian food
I'm so sorry. :(
If you happen to select veg that is actually available local and in season, you can get it fresh. That means you have to be a bit selective; the most premium example is probably asparagus. Lots of UK veg growing is in nearby counties like Kent.
The British reputation for disastrous food is mostly a legacy of rationing combined with a slightly colder temperate climate, and ceased to be representative in the 90s.