A tradition to fry in nut oil since when? The best fish and chips have always, and still are, been fried in beef dripping.
Hull has always fried in nut oil until the 70s when lard started infiltrating from elsewhere.
Presumably it was either easier to supply, cheaper or just more popular than other methods, and I like to think Hull people and fish shops knows its stuff on fish and chips. Just because you aren't aware of our traditions doesn't make them never have happened. I think you'll find the fish and chips have gone downhill since the 70s too, which says a lot about lard.
Bob Carver still fries in nut oil and it hasn't done his family any harm over the last century, although I appreciate he isn't a shining example of decent fish and chips these days.
