You've left me rather confused because your question heading refers to 'ABH' but you then write about a charge of 'GBH'. Which is it?
If it's ABH, the table on page 17 of this document applies:
http://www.sentencing...inst-the%20person.pdf
For GBH see page 15. For GBH with intent see page 13.
The sentences there are those which judges are obliged to adhere to unless there are very exceptional circumstances (which a judge must explain in open court). They refer to first-time offenders convicted after a trial. An early guilty plea can see a reduction of one third of a sentence. Previous convictions can push the sentencing range higher. Acting as part of a group will be seen as an aggravating factor, and will push the sentence towards the upper end of the relevant range.
Chris