'No previous'? What happened about the charge you faced earlier this year then?
http://www.theanswerbank.co.uk/Law/Question1394404.html
As I read it, it will be treated as a 'Category 2'offence because 'harm' will be classed as 'high' (because of the fact that the offence is regarded as being close to GBH) but 'culpability' will be regarded as 'low'.
That means that a Magistrates Court has to work up or down from a 'starting point' sentence of 26 weeks imprisonment. The court can pass a sentence as low as a 'low level' Community Order or, if they feel that a sentence of greater than 26 weeks needs to be passed, refer the matter up to the Crown Court (where a sentence of up to 51 weeks imprisonment can be passed). However those sentences can be reduced (by up to a third in the case of custodial sentences) for an early 'guilty' plea.
See pages 201 to 204 here:
http://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG_web_-_October_2014.pdf
(which,BTW, is the same link that I gave you earlier this year).
So custody is certainly a possibility but the courts usually try to keep people out of prison wherever possible, so you still stand a reasonable chance of avoiding being 'banged up'.