Preheat the oven to 180°C (350°F) and prepare your frying oil by heating vegetable oil in a deep fryer or large saucepan to 180°C (350°F).
Peel and cut the maris piper potatoes into thick chips. Rinse under cold water to remove excess starch, then pat dry with a clean towel.
Blanch the chips in the hot oil for about 5 minutes until they are soft but not coloured. Remove and drain on kitchen paper.
In a large bowl, mix the 00 flour, baking powder, and salt. Gradually whisk in the stout until you have a smooth batter.
Dry the cod fillets with kitchen paper. Dip each fillet into the batter, allowing the excess to drip off, and carefully place into the hot oil. Fry until golden brown and crispy, about 5-7 minutes. Remove from the oil and drain on kitchen paper.
Increase the oil temperature to 200°C (390°F) and return the blanched chips to the oil. Fry until golden and crispy, about 3-5 minutes. Drain on kitchen paper and season with salt.
Serve the beer-battered fish and chips hot with lemon wedges for squeezing over the fish.