Prepare the chips: Peel potatoes, cut into thick strips, rinse, boil 3–4 minutes, pat dry. Fry at 300°F (150°C) for 4–5 minutes until soft but not golden. Set aside.
Make the batter: Whisk flour, baking powder, salt, and pepper. Slowly add cold sparkling water (or beer) until smooth. Refrigerate 15 minutes.
Fry the fish: Pat fillets dry, dust with flour, dip in batter. Fry at 350°F (175°C) for 6–8 minutes until golden. Drain on wire rack.
Finish the chips: Fry at 375°F (190°C) for 3–4 minutes until golden and crispy. Salt immediately.
Serve: Plate with malt vinegar, tartar sauce, lemon wedges, and mushy peas.