Employed or Self Employed

My client is a small retail shop on a Scottish island. He  trades only between Easter and the end of October each year and requires typically two people to operate the shop during the season. His "employees" want to be treated as self employed - apparently another larger business on the island in the hotel trade engages all of its workers on this basis. I maintain that in a retail environment it is almost impossible to satisfy the conditions of self employment: opening hours are fixed, the price is hours dependent. Am I missing something - like a commonly accepted practice - or am I right to insist on the workers being treated as employees?