alfaOnly food I hated in EU was at i55 where the pubs + restaurants in that city were just garbage. London however, had A+ food.
The problem is a lot of ordinary people in the UK couldn't give a shit about proper, good food. Our food culture is rubbish, I know loads of people for example that won't touch fish or seafood (except for battered fish & chips) and think a good portion of vegetables is frozen peas and carrots boiled to mush.
As such a lot of people, when they go out to eat, just want a plate of cheap, filling and very boring food and so that's what a lot of our pubs and restaurants cater for. You can't really just walk into a place you don't know on the high street and expect to get served good food there unless you get really lucky (or you happen to be in a trendy up-market area, hence your positive experience in London).
Similarly our supermarkets tend to be shit, especially the limited selection of pretty boring veg and lacklustre seafood on offer.
What we do have, however, is some of the best produce in the world. Quality of meats, dairy/cheeses (fight me France, our cheese is better), seafood, beers, ciders are second to none. We send an enormous amount of top quality seafood to markets in Europe where the people actually appreciate it. And although we don't have the climate for citrus fruits, olives, etc. the fruit and vegetables we can grow are great. I was stuffing my face with some amazing locally-grown asparagus earlier tonight.
It's absolutely a case of knowing where to look. Visiting local markets or fishmongers rather than supermarkets, or knowing that one amazing foody/craft ale pub in the area, or getting a recommendation of a quality restaurant from a local who is actually interested in decent food.
Tl:dr UK has great produce (and therefore great food if you know where to look), but awful food if you don't.