The magistrates have to consider both 'harm' and 'culpability'. If both factors are classed as 'high', it's a Category 1 offence. If only one of those two factors are 'high', it's Category 2. If neither factor is 'high', it's Category 3.
In the type of situations usually involving door supervisors (where, for example, excessive self-defence was used or the door supervisor had been subjected to a great deal of provocation), 'culpability' is likely to be 'low'. However headbutting a victim, or kicking him/her in the head, can result in 'culpability' being viewed as 'high', as can other factors such as acting in response to the victim's sexuality.
For 'harm' to be seem as 'high', the injury/ies caused would need to be relatively serious as far as ABH offences are concerned (i.e. close to 'GBH') or possibly to have resulted from a prolonged attack.
Someone convicted of a Category 1 offence would definitely be given a custodial sentence. (The 'sentencing range' is between 1 and 3 years' custody). However a sentence not exceeding 2 years can be suspended, which would almost certainly happen in the case of a first-time offender.
Someone convicted of a Category 3 offence CAN'T be sent to prison. (The 'sentencing range' runs from a small fine up to a 'high level' community order).
Category 2, unsurprisingly, fits in the middle, with a 'sentencing range' anywhere between a 'low level' community order and 51 weeks' custody. (Once again though, any custodial sentence would almost certainly be suspended for a first-time offender).
Without knowing how the magistrates might rate both 'harm' and 'culpability', it's impossible for anyone here to give any firm predictions. However, having studied court reports for over half a century, I can say without the slightest shadow of a doubt that, if convicted, you WON'T be going to prison. The most likely outcome would probably be some sort of community order or even just a fine.
See page 177, et seq, here to see exactly what the magistrates would have in front of them if you were convicted of ABH:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf