"Fish and chaps" sounds close to "fish and chips" which is a popular British dish that is composed of fish and french fries. Chaps is another word for man, or boy. A cannibal is a mammal that consumes its own species, in this case a human that eats humans. This joke is playing on the popular dish "fish and chips" except "chips" was replaced with "chaps" because the joke is talking about a cannibal.
The British cannibal enjoyed snacking on fish and chaps .