I enjoy seeing known faces pop up in programs before they really found fame. Two recently were Martin Clunes (Doc Martin) in a 1992 Morse episode called Happy Families. Now I begin to see the resemblance between him and Boris Johnson - in fact I wondering if he is playing that role as well.
The second was even more unlikely. Peter Sallis is in the 1971 4th episode of Budgie series 1. He is even wearing the same clothes as he did later in Last of the Summer Wine, and he is playing a Northerner. (In fact according to Wikipedia he is a Londoner.)