Make the tartar sauce:
In a small bowl, combine all of the ingredients, adding salt and white pepper to taste. Cover and refrigerate until ready to use, up to 1 day ahead.
Make the fish and chips:
Fill a heavy-duty 6-quart pot about one-third full with oil. Heat the oil over medium heat until it reads 260°F on a deep fat/candy thermometer. Add half of the potatoes and cook, stirring occasionally with a skimmer or a large slotted spoon, until tender (you should be able to cut one with the side of the spoon) but not colored, about 5 minutes. Transfer the chips, shaking them to drain excess oil, to a paper-towel-lined baking sheet and spread in a single layer. Let the oil return to 260°F and repeat with the remaining potatoes.
Put a small rimmed baking sheet in the center of the oven and heat the oven to 250°F
In a medium bowl, combine the flour, baking powder, 1 tsp. salt, and the water. It will have the consistency of pancake batter. Put the batter in the refrigerator and use within 20 minutes.
Raise the temperature of the oil to 325°F over medium-high heat. Pat the fish dry with paper towels and season with 1 tsp. salt and 1/2 tsp. pepper total. One at a time, dip two of the fillets in the batter and turn to coat well. Allow excess batter to drip off. Transfer the fillets to the oil, and fry, flipping once with tongs or a slotted spoon, until crisp and golden-brown, 7 to 8 minutes. Transfer the fillets to a paper-towel-lined plate to drain and then transfer to the baking sheet in the oven to keep warm. Let the oil return to 325°F and repeat with the remaining fillets.
Raise the temperature of the oil to 375°F over medium-high heat. Add a small batch of chips and cook until crisp and golden, about 2 minutes. Using the skimmer or slotted spoon, shake the chips to drain excess oil and transfer them to another paper-towel-lined baking sheet. Sprinkle with salt. Let the oil return to 375°F and repeat with the remaining chips, cooking them in small batches. Keep them warm in the oven as well, but do try to serve everything within about 10 minutes of frying.
Serve the fish and chips with the tartar sauce, lemon wedges, and malt vinegar.
You can “blanch” the chips (cook them at 260°F) up to 2 hours ahead; save the oil in the pot off the heat and keep the chips in a single layer on paper towels.