Mix together the cornflour and polenta. Place in a large freezer bag and season well. Cover a baking sheet with kitchen paper.
Fill a large, deep saucepan one-third full with oil and heat until a cube of bread browns in 15 seconds. Add the squid to the bag and shake well until coated. Shake off the excess cornflour mixture and deep-fry for about 3 minutes, or until just golden and cooked through. Place on the prepared baking sheet and keep warm. Repeat, each in turn, with the prawns, scallops and fish. Serve with lemon wedges for squeezing over.