(2-part post):
The Crown Prosecution Service have 'charging standards' which indicate the appropriate charge for different levels of assault. The bad news, from your point of view, is that a compound fracture of the jaw takes an assault into the category of 'grievous bodily harm':
http://cps.gov.uk/legal/section5/chapter_c.htm l
When considering sentencing, courts have guidelines which provide them with a 'starting point' to work from. They then take relevant factors into account, to see whether there is justification for being more lenient or going beyond the guideline sentence.
The 'starting point' for a GBH offence will always be a custodial sentence but, in your case, a good barrister might well be able to convince a court that there is no need for you to go to prison.