The Sentencing Guidelines Council has produced a draft document regarding sentences for assault. Even though it's currently only a set of proposals, it's likely that it closely mirrors existing court practices.
The bad news (from your point of view) is that the 'starting point', when a court considers sentencing for a Section 20 offence, is always a custodial sentence. The good news is the inclusion of these two sentences in the draft document:
"Matters of personal mitigation are often highly relevant to
sentencing for this offence and may justify a non-custodial sentence, particularly in the case of a first time offender. Such a disposal might also be considered appropriate where there is a guilty plea".
You can read the full document here:
http://www.sentencing-guidelines.gov.uk/docs/A ssaultandotheroffences%20Consultationguideline FINAL-2007-06-21-JS.pdf
With a good barrister (and taking into account overcrowded prisons) you stand a reasonable chance of getting a non-custodial sentence but it would still be wise to pack a bag just in case things don't go your way. See the last two paragraphs of my multi-part post, here, for some suggestions as to what you should take with you:
http://www.theanswerbank.co.uk/Law/Question444 382.html
Chris