Magistrates and judges divide assault cases into 3 categories. If they view both 'harm' and 'culpability' as 'high' it's a 'Category 1' offence. If only one factor is seen as 'high', and the other as 'low', it's 'Category 2'. If both are seen as 'low' it's 'Category 3'.
If the magistrates see your offence as 'Category 3' you can't be sent to prison.
If it's 'Category 2' the magistrates have to start off by considering a prison sentence of 26 weeks and then work up or down from there. They can go as low as a 'low level' Community Order or as high as passing the case to the Crown Court (where a sentence of up to 51 weeks can be passed). Previous convictions for violence might well see the magistrates unprepared to consider a non-custodial sentence.
If the offence is classed as 'Category 1' then the magistrates must transfer the case to the Crown Court where, unless there are very exceptional circumstance, the Court must pass a custodial sentence (of between 1 and 3 years).
So you need to work out which category your offence will be seen as falling within. See pages 201 to 204 to read exactly what the magistrates will have in front of them when sentencing:
http://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG_web_-_October_2014.pdf
See pages 11 to 14 here to see what a Crown Court judge would have to refer to (if the case reached that court):
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf