Our local chippy sells all sorts of sweeties battered and deep fried, from individual Opal Fruits (or whatever they're calling them now) to Lion Bars and Creme Eggs. If they don't stock the sweetie themselves and you take it in they'll batter and fry it for you. I've also seen battered and deep fried pizzas, though that was through in Weegie-land. Prior to diagnosis, and in the interests of scientific research, I once had a bite of a deep fried Mars Bar. It was every bit as disgusting as you'd imagine.
Been veggie for the best part of thirty years and can no longer remember what chip shop fish tastes like. My fave chippie fare was Cheese and Onion Fry-Its, particularly on a buttered breadcake with lots of vinegar. I think that would be the best part of a day's carbs now :lol:
As for the business idea why not do some market research amongst the other punters next time you're there? Asking them or passing out flyers for a computer survey or even asking on a petrol head message board? Years ago I went to a political festival in Paris, the food was fabulous, definitely made the whole event better in my eyes. I tend to agree about widening your target audience, you'll do better with a wider selection of fresh food of which some was low carb than all low carb. The labelling would seem to me to be the key thing.