King Charles' head is a reference from David Copperfield. Whenever Mr. Dick, one of the characters, tried to write anything sooner or later he would find himself mentioning King Charles the First's head, however hard he tried not to.

"King Charles's head" is used by some people to mean an obsession or distracting idea that continually interferes with whatever you're supposed to be doing.

Bingley

Bingley