While there are calls for the law on 'joint enterprise' to be changed, as things stand if you're part of a group where a person suffers injury through the actions of that group, then you're all guilty of the assault.
A broken jaw is at the lower end of the type of injuries that fall within GBH, so a judge would almost certainly rate 'harm' as 'low' when considering sentence. Having a 'subordinate role in a group or gang' means that 'culpability' would probably also be rated as 'low' too, so that makes it a 'Category 3' offence. Even so, the absolute minimum sentence that a judge can pass (other than in really exceptional circumstances, such as a terminally ill offender) is one of 3 years imprisonment after a trial or 2 years imprisonment with an early 'guilty' plea.
So your legal representatives need to fight hard to try to get the Section 18 charge against you dropped but they might struggle to do so. If they can't, you're definitely going to prison.
This is the actual document that judges must refer to. See pages 3 to 7:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf