Mix soda water, flour, egg and salt and pepper in a bowl. Parboil potatoes in salted water for about 5 minutes, until tender. Drain and pat dry with a paper towel before deep-frying in batches for about 5 minutes, until golden and cooked through. Coat fish in the batter and deep-fry in hot oil for 2-3 minutes, until golden and crunchy. Remove fish and chips from hot oil with a slotted spoon and drain on kitchen roll to keep them crispy. Sprinkle with flaked sea salt and serve with a squeeze of lemon.