The reason for this curious message? The United Nations weren't very happy that people could associate their very real organization with a fictitious one, and the producers decided a tongue-in-cheek name-check to UNCLE in the end credits would throw gullible viewers off the scent. And it was needed, too: many, many people thought that UNCLE was real, and the show received letters from viewers who wanted to join up!
Taken from 9 things you need to know about MFU
When you saw that, did you think UNCLE was real?
Did you ever try to join?