More than likely just for some publicity for a tv show or something as its his 21st birthday this weekend (the real prince, that is) They used the same guy for a Sky One show on last week about the Princes life as he approaches 21.
Probably that Matthew Turpin lookalike chap. He does look amazingly like William to be fair. Must have been doing some pr stunt/tv show etc regarding the real William's 21st.