I never heard of slaphead, so I googled it. It does seem to be mostly British. It has a few entries in the Urban Slang dictionary, which puts it ahead of the variant that I tried, headslap. Headslap has only 960 entries in Google, which makes it pretty darn obscure. A headslap is when an American football player hits his opponent alongside the head with his hand (or fist), a move that is now illegal. A few people (apparently) use the term to mean "reality check."
