agree with LEAVE if u do fight, u dont know how many more fat bastards will waddle out of the pub to back him up. Saying that, if he;s an obvious twat its unlikely he'll have mates who care enough. always aim for the face/temples as an initial smack target as well. I'v previously got out of that situation by being unreactive and keeping calm eye contact.
"I;m going to mess u up, I dont like you;re face.."
>"Oh really, thats a shame"
"I;m really pissed off about XYZ"
>"Not my problem"
"What!?"
>"Thats not my problem"
"If there werent cameras here I'd break ur face"
>"Oh ok, see you mate, cheers"

felt pretty pleased with outcome. incidentally, have u seen in fights where girlfriend is present she will often kiss her bf infront of agressor as if to say "look, this guy is high value, he has a woman, she is attached to him and unavailable, his RAS is on her not the agressor". It never fucking stops a fight but its an interesting behaviour.