Judges are given a 'starting point' and a 'sentencing range' which (unless there are very exceptional circumstances) they're meant to stick to.
The starting point for sentencing an assault by a first time offender, causing grievous bodily harm, where the assault was not premeditated, no weapon was used and the injury was not 'particularly grave' is 24 weeks custody .
The sentencing range is from a community order (at the highest end of the scale) up to 36 weeks custody.
That, in itself means that there is at least some prospect of a non-custodial sentence. However, those guideline sentences apply to offenders convicted after a trial. An early guilty plea will skew the sentencing range towards the lower end of the range, which further increases the chance of a non-custodial sentence.
Given the present level of overcrowding in Britain's prisons, together with the 'impeccable references', it's probably most likely that a non-custodial sentence will be passed, but a short period of custody can't be completely ruled out.
Chris