human head vs bonce

human head and bonce both are nouns.

Word	Noun	Adjective	Verb	Adverb
human head	Yes	No	No	No
bonce	Yes	No	No	No

As nouns, bonce is a hyponym of human head; that is, bonce is a word with a more specific, narrower meaning than human head:

Other hyponyms of human head include bullethead, attic, bean, dome, noggin, noodle.

human head (noun)	bonce (noun)
the head of a human being	informal terms for a human head