Quite correct, it's the least serious assault charge. S.39 of the Criminal Justice Act 1988 provides for common assault being a summary only offence, i.e. only triable in the magistrates court (can't go to Crown Court).
Maximum sentence for common assault is 6 months imprisonment or a fine up to 5000 pounds. Guilty please give credit, i.e. a lesser sentence but it sounds like you will not be pleading guilty. Please be aware that self-defence can be tricky and practically you have to satisfy the court that any action you took to defend yourself was entirely proportionate to the threat you faced.
If convicted, sentencing for a first offence could be as low as a conditional discharge (requirement to be of good behaviour for a period, 12 months usually), or a fine. If the matter is a domestic issue, usual sentence is a community order with the requirement to undertake anger management etc. Custody is rare for common assault, usually only if it's a worrying assault, i.e. on a vulnerable person, or there is a long history of offending behaviour.
Provided it's not domestic or particularly serious and you have no previous history of offending, I'd expect a conditional discharge or fine.
Hope that assists, feel free to ask anything else that occurs.