Pour 2 inches of oil enough oil into a wide heavy pot to reach a depth of 2 inches. Heat the oil over and heat over medium heat until a deep-fry thermometer registers 375 degrees F. Pour some flour into a shallow dish or bowl. Dredge 1 fillet at a time into Working with one filet at a time, dredge fish inthe flour, shaking off any excess, and then dip in the batter until. Dip fish into batter until completely coated, then letletting the excess drain off. Slip the fish flat into the hot oil; don''t just drop it in or it will curl up. Fry the fish, turning once, until golden brown, 3 to 4 minutes. Transfer to a wire rack to drain.
Lower heat until the oil temperature reduces toto 345 degrees F (or have a separate pot of oil heating). Working in batches, blanch the potatoes in the oil just long enough to soften the insides. Drain, Ttransfer to a paper towel-lined tray or plate to drain, and let cool completely. Increase the heat until oil temperature reaches to 370 degrees F. Again, working in batches, fry the potatoes until golden brown. (British-style fries are soggy, not crisp.)
Serve the cod with fries, salt and malt vinegar.