Pillock.

One of my favourite words because of the way it sounds and the way its sound matches its meaning. Also because it can be used with a wide range of meaning:

"You PILLOCK!" = "You absolute idiot, it'll take ages to fix that!"

"Tch! Pillock!" = "My, my, you have been a silly person, haven't you?"

"you pillock" (with a smile) = response when someone tells about a minor lapse of judgement that they've just made.

And so on....