Mate, while I wouldn't want to face the barrel of one, it's important to remember that the limit on air rifles in this country has been set specifically to limit their effectiveness as a weapon. I imagine that it'd sting like a hornet, but I doubt it penetrate a thick puffa-jacket or coat, and even then it wouldn't incapacitate your assailant; probably just fuel their rage.
It's important to remember that firearms aren't a pain-compliance weapon, they're designed for instant incapacitation and the very minimum force required for this was conjectured by the American army to be around 60 ft