Defrost the prawns for 1 hour. Peel off the shells, leaving the last section of tail and tail fins on.
Sieve the flour and salt for the batter into a small basin and hollow out the centre. Add the oil and water. Using a wooden spoon, mix from the centre outwards, gradually drawing in the flour from around the sides of the bowl. Beat well to get a smooth batter. Whisk the egg white until stiff and, using a metal spoon, fold into the batter.
When ready to fry, dip the prawns by the tail ends into the batter. Allow excess batter to drip away and add the prawns a few at a time to the hot oil. Deep-fry for 2–3 minutes or until golden brown. Drain and tip out on to absorbent kitchen paper. Keep the fried prawns hot in the oven, uncovered so the batter remains crisp, while frying the rest. Serve with wedges of lemon and tartare sauce.