Instapundit linked to an article by Joyce Lee Malcolm contrasting the right of self-defense in the U.S. and Britain.
Short version, in Britain you can't.
Or you can, but you'll probably go to jail for it. Even if they attack you in your home, or with a weapon and you're unarmed, etc. Hurt your attacker, and you're screwed. Scare your attacker badly, you're screwed.
I repeat from Smallest Minority, "Stick a fork in Britain, they're done".