An assault is committed when a person intentionally or recklessly causes another to apprehend the immediate infliction of unlawful force.
Where battery results in injury, a choice of charge is available. The Code for Crown Prosecutors recognises that there will be factors, which may properly lead to a decision not to prefer or continue with the gravest possible charge. Thus, although any injury that is more than transient or trifling can be classified as actual bodily harm, the appropriate charge (subject to Para (viii) below) will be contrary to section 39 where injuries amount to no more than the following:
Grazes;
Scratches;
Abrasions;
Minor bruising;
Swellings;
Reddening of the skin;
Superficial cuts;
A 'black eye.
http://www.cps.gov.uk/legal/section5/chapter_c .html
I think you're preety lucky to be being charged only with common assault, I would imagine that unless you could show that you were facing an armed assailant it would be almost impossible to plead self defence sucessfully.
Common assult has a recommended sentence of a community penalty for a first time offender pleading guilty. But you could get 6 months if you pi$$ them off.
Mitigating factors include "impulsive action, single blow"
My advice to you is to plead guilty and stress your previous good character and that it was a momentry impulse that you deeply regret.
A lawyer with experience will be able to put this across much more effectively.
Whatever you do don't stand up and say "He was acting aggressively so I nutted him in self defense"!