Unless there are genuinely exceptional circumstances (which a judge must explain in open court) all judges have to keep to the guidelines produced by the Sentencing Guidelines Council.
For the lowest level of 'Section 20' offences (where there was no premeditation, no weapon used and no particularly grave injury or disfigurement) the 'starting point' is 24 weeks custody. Working up or down from that point (depending upon the circumstances of the case), the judge has to arrive at a sentence no higher than 36 weeks custody and no lower than a 'high level' community order.
However those sentences refer to a first-time offender convicted after a trial. An early guilty plea can see a reduction in the sentence (which, in the case of a custodial sentence, is normally by one third).
Additionally, 'provocation' can be seen as a mitigating factor.
With a good barrister to speak for you, possibly pointing out the effect that your conviction might have on your career, it's likely (but not completely certain) that the judge will opt for a non-custodial sentence and instead impose a community order. (In the fairly unlikely chance of you being 'sent down' it's also worth noting that the time served in prison is normally only a half of the actual sentence. So even if the judge stuck to the 'starting point' of 24 weeks custody, he'd cut it to 16 weeks because of your guilty plea and you'd only actually be 'banged up' for 8 weeks).
Chris