In fact, "Fuck off" is not an insult anywhere. It's an obscenity, but it doesn't fulfil the criteria for an insult, in that it does not denigrate the person of the hearer in any way.
"Wanker" is an insult. "Bastard" is an insult. "Your father was a hamster and your mother smelled of elderberries" is an insult.
"Fuck off" is an instruction.