Classic British Fish and Chips

Name: Classic British Fish and Chips
Prep time: 25 min
Cook time: 35 min
Total time: 1 h
Cuisine: British
Ingredients: 7 tablespoons (55 grams) all-purpose flour (divided); 7 tablespoons (55 grams) cornstarch; 1 teaspoon baking powder; Sea salt (to taste); 1 pinch black pepper (to taste); 1/3 cup dark beer (cold); 1/3 cup sparkling water (cold); 4 (7-ounce) fish fillets (thick, white fish); 2 pounds potatoes (peeled); 1 quart (1 liter) vegetable oil (or lard; for frying )