The fresh fish stall at my local market closed about 3 years ago
Most supermarkets sell fresh fish, but those departments are also closed at the moment
They have packaged, chilled fish, and Morrisons seem to have a better selection than most
There is also Iceland
They have to cater for all tastes, so a lot of their fish is battered or packed in sauces, but if you search you can find plain fish
I have had sea bass, mackerel fillets, various shrimps & prawns, plain white fish, and fish pie mix, which I use in stir frys
I'm sure there will be others
They also do a variety of packs of vegetables; mixed berries, and so on. all frozen of course