Anyway my feelings are a little bit hurt when you say that you find it in posh neighbourhoods. I'm not posh! I'm just Greek and it seems, like you, I'm used to a highest standard in food quality.
I guess it's a huge privilege to have grown up with good quality food not just sufficient calories- and being able to eat good food a few times a year still. I freak out a bit when I think how much we can lose with climate change. But that's another discussion.
Anyway this is why I don't like to accuse the Brits of having a crap cuisine (or eating too much meat). They do, but that's because their (other) materials are not very good to begin with. If you think about it, the only stuff that grows locally are cabbage, turnips, potatoes and apples. You can only go so far with that.
I'm sure that's a major reason for all the problems this country has. Food is one of the biggest things in life, up there with companionship, sex, good wine and natural surroundings. If you don't get good food, there will always be something missing from your life.
Try the Waitrose. I'm Greek and the best supermarket taramosalata I've ever had is from Waitrose. To clarify home-made taramosalata is cream coloured, but taramosalata sold at supermarkets is always dyed bright pink, for some reason. And that includes Greek supermarkets. The only supermarket taramosalata I've ever seen that has the right colour is the Waitrose one- and it also has the right flavour and consistency. Well ish. Nothing can ever beat my yiayia's taramosalata, but it's a good approximation.
I assume their other "ethnic" foods are also of similar good quality (for mass-produced stuff). They also have sort of decent fruit and veg and their own brand of Duchy Organic farming produce (milk, eggs, meat, fish, etc) that is actually good.