Marks and Spencer is a company that is generally regarded as a bit more “up market”. If you go into their stores, the staff are generally friendly and smart, and that’s what they’re looking for in new employees.
Customer service is crucial for M & S as a business, and that’s probably why they have the role playing during interviews.
Like girlythts mentioned, there may be other scenarios given to you, so practice thinking on the spot in case they give you one that you haven’t rehearsed!