Rinse the chips thoroughly under cold running water. Place in a large pan of salted cold water and bring to the boil over a medium heat. Cook for 4–6 minutes until just tender. Drain well and set aside on kitchen paper to dry completely.
Pat the fish fillets completely dry with kitchen paper. Squeeze a little lemon juice over each fillet and season lightly with salt and pepper. Dust each fillet lightly all over with plain flour, shaking off any excess.
Sift the plain flour, sea salt, baking powder, and cayenne into a large bowl. Whisking continuously, gradually add the cold beer until you have a thick, smooth, shiny batter. You may not need all the beer — stop when the batter coats the back of a spoon thickly.
Bring just enough water to cover the peas to the boil in a small saucepan. Add the peas and cook for 2–3 minutes until tender. Drain, reserving a little cooking water. Transfer to a food processor with the butter, one tablespoon of reserved cooking water, the mint, and chilli. Pulse to your preferred consistency. Season well and keep warm.
Heat enough oil for deep-frying in a deep-fryer or deep heavy-based saucepan to 180°C — no more than two-thirds up the pan. Test with a drop of batter: it should sizzle and crisp immediately.
Pat the parboiled chips completely dry with a clean tea towel or kitchen paper. Carefully lower into the hot oil and fry for 3–5 minutes until just beginning to colour. Remove with a slotted spoon, drain well on kitchen paper, and set aside.
Dip each floured fish fillet into the batter, turning to coat completely and letting any excess drip off. Carefully lower into the hot oil one or two at a time. Fry for 6–8 minutes depending on thickness until crisp and deeply golden. Remove, drain on kitchen paper, season with salt immediately, and keep warm.
Return the par-fried chips to the hot oil and fry for a further 3 minutes until deeply golden and crisp all over. Drain well on kitchen paper and season generously with salt the moment they come out of the oil.
Plate the battered fish hot alongside the triple-cooked chips and a generous spoonful of mushy minted peas. Serve with lemon wedges or malt vinegar and your favourite dipping sauce.