You've missed out an important piece of information. i.e. what charge are you facing? Based upon what you've written I'd guess at 'Section 20 GBH [or 'Wounding', rather than 'Section 18 GBH with intent' or 'Actual Bodily Harm. The differences between those offences are described in his document from the Crown Prosecution Service:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/
Assuming that it's 'Section 20' the judge would have to consider both 'harm' and 'culpability' when sentencing. If he/she regards both of those as 'low' it's a Category 3 offence, where the 'starting point' sentence is NON-CUSTODIAL. (It's a 'high level' community order). The maximum sentence for such an offence is 51 weeks imprisonment (reduced to 34 weeks for an early guilty plea, with only half of the nominal sentence actually spent 'inside').
My guess is that 'Category 3' will apply and that you'll then almost certainly avoid prison. However if either 'harm' or 'culpability' is regarded as 'high' then it becomes a Category 2 offence, where the judge has to pass a custodial sentence (other than in very exceptional circumstances). See pages 7 to 10 here:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
(That's the actual document that the judge will refer to in court).
Chris