I have jus been resarching this very situation in UK law. It boils down to three things,
1/ You must show that you reasonably fear immediate attack, or you are being attacked
2/ You must show that there was no other reasonable alternative, bearing in mind "the heat of the moment"
3/ The response must be proportional to the threat. If you are attacked with a balloon it's very difficult to prove proportinality if you use a knife.
The problems arise when the court finds out you are a martial artist, everyone who has just walked past a Dojo will be described in court as a martial arts "expert" and, therefore, as someone who should have known better or was just looking for an excuse to use their training.
The main thing is try not to ever get into a situation where you have to use your knowledge, because the DLS that attacked you will get a better lawyer and will drag you through the courts. Unfair I know, but that's how it is.