Our closest chippy (Whitstable, Kent) is £13.50 for a large cod and chips, and whilst not the cheapest, it is the best in town – the batter on the fish is crispy and not too greasy and the cod inside nice and flaky but not overcooked. The chips are always crisp outside and fluffy inside, and they clearly use clean oil as you don’t get any kind of stale oily flavour with them. My only slight complaint is the mushy peas are always a bit too runny and under-seasoned, but the mains are always good. So generally I’ll do my own mushy peas at home, and just bring back the fish and chips.
As for supermarket battered fish, the best one I’ve tried that I can think of is the Sainsbury’s Taste The Difference beer battered cod. Not the biggest portions you’ve seen, but they’re chunky and the batter is light with a good “beery” flavour.