All fish is OK for diabetics, as long as you're OK with fish anyway, but as mentioned, beware of the accompaniments like sauces, chips, and so on
Though a small plain new potato here & there might be OK, check with your meter to see how that affects you
The fish monger stall at my local market closed years ago, and since Covid the fresh fish & delicatessen counters at supermarkets closed, and don't seem to be re opening, at least, nowhere near as extensive as they were
So mostly now I use frozen fish from Iceland (the store, not the country!) which suits me as I live on my own so I can stock up then eat what I need
Recently I got mackerel, sardines, and fish pie mix on Iceland's 3 for £10 deal. They have others, including a general mixed seafood pack
I use the fish pie & general pack in stir fries
Morrissons is slightly more up market and they sometimes have a better range of fish, but I haven't been there for a while
I also eat canned fish - sardines, mackerel, tuna, and so on. I prefer canned fish in oil or brine, but not tomato sauce
Sainsbury sell Burgen bread, and probably others, but you have to search for it amongst all the other more common types
You can do something cheap & simple like cabbage with mackerel or kippers; or things like Mediterranean type sardine salads, & salad Nicoise.