>>>if found guilty what can i expect. this is my first offence
When sentencing 'Section 20' offences, judges have to take into account both 'harm' and 'culpability'.
'Harm' will be regarded as 'low' if the level of injuries sustained by the victim only just fall within those that qualify as 'GBH'. It will be regarded as 'high' if it was life-threatening or resulted in significant permanent disability or scarring. (It will also be regarded as 'high' if the victim was particularly vulnerable or suffered severe psychological problems, or if the attack was prolonged and sustained).
'Provocation' or 'excessive self-defence' can see 'culpability' regarded as 'low' but the use of a 'weapon' (which can include head-butting or kicking with a shod foot) can see it regarded as 'high'.
If BOTH 'harm' and 'culpability' are seen by the court as being 'high', it's a Category 1 offence, with a sentence of between 2 years 6 months and 4 years custody for a first time offender (which can be reduced by up to a third for an early guilty plea).
If ONE of 'harm' or 'culpability' is seen as 'high' (with the other being seen as 'low'), the sentence is one of between 1 and 3 years custody (again with a reduction for an early guilty plea).
If BOTH factors are seen as 'low' the sentence is one of between a 'low level' community order and 51 weeks imprisonment (with reductions for an early guilty plea).
Anyone sentenced to a term of imprisonment of less than 4 years is normally released (on licence) half way through their sentence, or possibly slightly earlier if a tagging scheme is deemed to be appropriate.
Source:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
Having typed all of that lot out, I'll point out that the courts usually do their best to keep first time offenders out of prison, so a custodial sentence would be unlikely if both factors mentioned above are seen as 'low'. Even with one of them being seen as 'high', a suspended sentence is still possible.
Chris