You can believe the references the Wikipedia article is based off, such as BBC Scotland:

"They surveyed hundreds of fish and chip shops in Scotland to find out if "the delicacy" was available and if people were actually buying them. It found 66 shops which sold them, 22% of those who answered the survey. [...] Annie Anderson, from the Centre for Public Health Nutrition Research at the University of Dundee, used to send her medical students out into the city to see if they could find somewhere that sold deep-fried Mars bar. "It was not much of a challenge in Dundee," she says."