I heard that there was one rude employee at one shop in Glasgow. Is that the custom in Scotland?
Of course, the tip is not required. Required means that it shows up on your statement. The fact that the employee "chased" you down suggests that he wanted a signature to authorize the room charge.
It is common practice by most people to leave a few dollars at a buffet and a full standard tip of 15-20% at a full service meal. This is the case whether the meal is paid for you by in cash or is included in your room rate in one way or another. The reason is quite simply that the staff don't really care how you come about to get fed, they still work for the same wages.
Making comparisons between countries and their customs is a fool's errand. Part of travel is adapting.
As with man