moisture!
most chippies buy frozen cod, which they thaw, as they thaw they drop a lot of water out, then they sit for a while drying a bit more, then they are floured before being battered and fried
leave your filleted fish on a plate in the fridge for an hour or 2 (even overnight) with a tiny sprinkling of salt, before you cooked em pat them all over with kitchen towel to soak up as much moisture as possible, leave till back up to room temperature, then cook em
the other factor for chip shop fish is they have huge great friers, so when they drop a fish in the temperature of the oil doesn't drop much so the batter cooks quickly, sealing in the fish, put too much in a chip pan at home and the temperature drops so much, that the fish is being boiled in oil as much as fried, which breaks down the texture of the fish, and can allow some of the flavour to boil away, rather than be sealed in the batter, try cook smaller portions of fish in batches
For what its worth, I never wash any fish that going in the freezer, it just adds more water, which when it freezes, expands and breaks down the flesh of the fish, freeze it unwashed, and give it a rinse when its thawed out again, keeps the flesh firmer