Actually the law permits the use of "reasonable force" to restrain someone who has or is about to commit a crime, this is not only for the police but extends to regular citizens.
I was gonna quote him, but you answered him for me.
I'll add this though (aimed at dynoryder), you would rather let a friend drink drive and possibly kill, say, a car carrying children, or any innocent people for that mattter, than belt your friend once or twice to prevent them driving and killing themselves and others?
I think you need to prioritise mate. I'd rather get charged with assault anyday than have a mate die and know they took innocent people down with them.