The judge has to look at two factor, 'harm' and 'culpability', in order to decide which 'category' the offence falls into.
'Harm' might be seen as 'greater' if, say, the injuries inflicted were particularly severe (perhaps requiring a lengthy stay in hospital), if they resulted in substantial psychological damage to the victim, if they resulted in significant life-long scarring or if the victim was particularly vulnerable for some reason. Otherwise 'harm' might be seen as 'lesser'.
'Culpability' might be seen as 'higher' if the offence came about due to homophobia or some other form of prejudice or if a weapon was used. [NB: The courts regard kicking someone, while wearing shoes, or headbutting them, as 'using a weapon']. Otherwise it might be seen as 'lower'.
If 'harm' is seen as 'lesser' and 'culpability' is seen as 'lower', then it's a Category 3 offence and you stand a very good chance of avoiding a custodial sentence (especially with an early guilty plea).
If 'harm' is seen as 'greater' but culpability is still seen as 'lower' (or 'harm' is 'lower' but 'culpability' is higher) then it's a 'Category 2' offence and (unless there are VERY exceptional circumstances) the judge MUST pass a custodial sentence. That might typically be around 12 months. (i.e. 18 months cut by a third due to an early guilty plea). However sentences not exceeding 2 years can be suspended, so you MIGHT still avoid prison.
If both 'harm' and 'culpability' are seen at the upper level then it's a 'Category 1' offence where, again the judge MUST pass a custodial sentence, with the chances of it being suspended probably being extremely slim.
'Section 20' offences are 'either way', meaning that the magistrates could have dealt with the matter themselves if they'd believed that their sentencing powers were great enough. The fact that they've sent the matter to the Crown Court means that they were of the opinion that a prison sentence of greater than 6 months is required. However judges don't always agree with magistrates, so it's not unusual for offenders sent to the Crown Court for sentencing to avoid prison. However it might still be wisest to pack a bag.
See pages 7 to 10 here to read exactly what the judge is required to refer to when considering sentence:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf