However your description of the victim's injuries suggests that an 'ABH' charge is more likely. Unless there are extremely exceptional circumstances, a judge is obliged to sentence a first time offender (convicted after a trial) to between a 'medium' Community Order and 26 weeks custody. The 'starting point' is a 'high' Community Order, with the judge moving the sentence up or down depending upon the circumstances. Since an early guilty plea pushes the sentencing range downwards, IT'S EXTREMELY UNLIKELY THAT YOU'D BE SENT TO PRISON IF YOU WERE TO PLEAD GUILTY TO AN 'ABH' CHARGE.
Read this to see the definitions of the different levels of assault:
http://www.cps.gov.uk...s_against_the_person/
This is the document which judges must refer to when considering sentence:
http://www.sentencing...inst-the%20person.pdf
'Self-defence' can be very hard to use as a valid defence. You would need to establish that your actions were the only ones possible under the circumstances. (e.g. if you felt threatened, why didn't you simply seek to run away? That's the only lawful means of 'self defence 'unless the circumstances prevented it). Let your solicitor guide you as to whether it's best to pursue such a defence, or better to enter a guilty plea to 'ABH'.
Chris