If using frozen fish, thaw completely. Season the fish with salt, and let it sit for at least 20-30 minutes (up to 1 hour) on a wire rack or paper towels in the fridge. This will help to extract the moisture from the fish and make it firmer for crispy fish.
Make the dredging flour. Combine flour with salt and pepper. Set aside.
Make the batter: In a large bowl, whisk the flour with baking powder, salt, pepper, and cold beer. The consistency of the batter must be the same as heavy/double cream (add more liquid or flour if needed to achieve this). Make sure not to over mix the batter.
Dredge the fish in seasoned flour and shake off any excess flour. It's important to get rid of any excess flour to get the right texture.
Heat the oil in a cast-iron pot with a kitchen thermometer, or in a deep fryer until it reaches 300°F (150°C).
Dip the fish in batter, take it out shaking any excess batter off. And lower down into the oil very slowly.
Deep fry for 6-7 minutes turning halfway through.
Remove from oil, and place on a wire rack for the extra oil to drip and for the fish to stay crunchy (it will stay crispy for 15-20 minutes, so best served hot).
Serve with chips, tartar sauce, and lemon juice or malt vinegar.