Knobhead is a mild swearword. Although knob means penis, it's one of the milder words for it (maybe one step worse than "willy" or "todger").

Note that wazzock, pillock, dipstick and plonker are all, at some place and point in time, also words for penis.

I prefer ‘helmet’ myself.

Yeah, I wouldn't allow it, personally. I find it's a bit of a weird one, like "pussy" - i.e. ruder than it should be, given that you can read the word in children's books.

A wizard's staff has a knob on the end

Isn't knob the one on the doors?