Its similar in Australia, reasonable force is OK, but anything more is not.
A recent magazine article I read by a karate student who is also a lawyer specialising in this said that kicking techniques are out totally unless you are outnumbered or they have weapons or you are getting your bum kicked.
One of our instructors was given a hard time by six guys, he broke the nose of the guy with the bigest mouth & pinned him with sankajo where he could see the others, who all decided that they weren't good enough. The police were there (public event) & didn't have a problem with this at all. According to our laws here it becomes reasonable because he thought the six guys were intending to do him injury & by taking out the first guy hard, he didn't need to touch the others.
On the other hand, there have been cases in the past where the judges have assumed that because you have martial arts eperience you are the expert in the incident & therefore shouldn't have needed to damage your assailant.