Place the breadcrumbs and coconut in a food processor and pulse together briefly to break up the coconut a little. Tip into a shallow bowl.
Season the cornflour with salt and freshly ground black pepper and place on a plate. Lightly beat the egg white in a shallow bowl.
Heat the oil in a deep fat fryer or deep, heavy-based saucepan to 180˚C (350˚F), or until a cube of bread browns in 15 seconds.
Meanwhile, toss the prawns, a few at a time, in the cornflour and shake off the excess. Dip them in the egg white to cover, then roll in the coconut mixture to make an even coating. Spread out on a large plate or baking sheet.
Cook the prawns in the hot oil for 3–4 minutes, lightly shaking to separate them, until golden brown. Remove them with a slotted spoon and drain on kitchen paper. Serve with lime wedges and sweet chilli dipping sauce.