If you were charged and brought before the Crown Court (because GBH with intent cases can't be tried in a Magistrate's Court) the jury would have to consider TWO matters:
Firstly, they would need to consider whether there was sufficient evidence to prove, beyond reasonable doubt that you had directly assaulted the victim. Obviously, if the answer was 'Yes' they'd find you guilty. However if the answer is 'No' they'd then have to consider whether you were part of a group intent on assaulting the victim, even though you never even touched him. If they decided that the answer to that question was 'Yes' then they should still find you guilty.
There are currently calls for the law on 'joint enterprise' to be changed but, as it stands at the moment, anyone who is part of a group which commits an offence (even if that group member doesn't directly carry out any criminal act) is guilty of that offence. By failing to disassociate yourself from the fracas you have put yourself at serious risk of conviction for an offence which carries heavy penalties.
Other than in very exceptional circumstances (such as the offender being terminally ill) judges must always pass an immediate custodial sentence for a 'Section 18' offence. The absolute minimum sentence which can be passed is 2 years imprisonment, based upon a first-time offender who pleads guilty to a 'Category 3' offence (which you seem to be describing), with all relevant factors weighing in the offender's favour. If that offender was convicted after a trial the minimum sentence would be one of 3 years. Previous convictions (especially for violence) can push sentencing higher.