See page 17 of this document:
http://www.sentencing...inst-the%20person.pdf
Unless there are very exceptional circumstances (which a judge must explain in open court) judges are obliged to stick to the guidelines you see there.
However those sentences refer to a first-time offender convicted after a trial. Pleading guilty at the earliest opportunity can see a reduction of one third in a custodial sentence (or an appropriate reduction in the level and length of a community order).
As the assault wasn't premeditated, the bottom line of the table seems to be the one the judge will refer to, so a community order seems to be the most likely sentence.
You're lucky that the victim's injuries were relatively minor. If the CPS had managed to get 'GBH with intent' to stick, you'd probably be looking at a sentence of 4 to 5 years imprisonment. (See the table on page 13 of that sentencing document).
Chris