To be honest with you Mark, even though it was extremely unfair and unjust by the 'boys', in the eyes of the Law you got out of the car, you went over to them and YOU attacked them. Rightly so too but Law wise you are the one in the wrong, and if I was you I would plead GUILTY.
I agree with a earlier statement, around here you were lucky to get arrested. It could of been a hell of a lot worse for your Daughter, you are a grown man and your actions are your own but think of your Daughter Mark, if you persue your innocence, will this not back fire on you from the 'boys'?
Depending on wether the guy needed hospital treatment or not, but from what you have said, I can imagine you would get a caution.. You have no other previouses that matter, and at worse a fine and community service as stated earlier.
But that's only if you plead Guilty. If you plead Non Guilty, I can imagine that you can end up with a helll of a lot worse. Is it worth takingt he chance knowing you were in the wrong.?
Frustrating I know because in your eyes they did deserve it, but a police approach may pay off better next time or even late at night get ya keys out and go looking for their car :-)
I wish you luck xoxo