See this is where you're going wrong. Baked potatoes and toasties are very quick and easy to make at home, and the baked potato in a cafe will highly likely be a reheated monstrosity. So not worth paying cafe prices for.
Whereas most people don't have decent deep fat fryers at home and a roast involves time and lots of peeling and chopping. Plus while they cost a lot more than they used to, both fish and chips and carvery are quite cheap for a cooked meal out. So I'd get one of those and have a toastie or a baked potato at home later.