The first time you hit him is probably excusable. You were preventing an assault against yourself. The law is getting much more sympathetic to people using defence these days. However, the second time was definitely a criminal assault (technically battery, actually, but I won't get into that, it doesn't matter). You weren't defending yourself this time, but responding to an insult. There is also no defence of provocation to battery.
So, what happens next?
I don't think criminal prosecution's likely. The guy wouldn't make a police complaint because he knows that he assaulted you first, so he'd be in as much trouble as you. If he did make a complaint, the police would review the evidence (CCTV and/or a statement from you, the man, and your manager) and you'd either get a caution or go to court. If you went to court, you'd get a light fine no more. (Unless you caused enough damage to be liable for GBH or ABH, unlikely).
If the police want to talk to you about this at any point, I suggest you say nothing at all (you can confirm your name and address, but that's all) until you say a lawyer. They'll provide the duty solicitor to you at the station. (This is all assuming the man makes a complaint).
All in all, the criminal side of this isn't much to worry about. Just don't talk to the police without a lawyer. Even if you get prosecuted, the worst you'll get is a court appearance and a fine.
As for what your employer would do, it's hard to say. Your actions would be grounds for dismissal, but your employer wouldn't have to sack you if they didn't want to. It would depend on your record, your relationship with your boss, company policy, etc, etc. The only way to know for sure is to call them tomorrow and find out what they intend to do.
If it's a small business, the likely outcome is that the whole thing will be dealt with by an informal talk with your boss and decided by him. If it's a larger company, you'll get a disciplinary probably but that's only if your manager reports the incident.
Good luck. On a personal note, it sounds like that guy totally deserved it. He sounds like an idiot. I wouldn't recommend doing that again, though :P