I'm unsure why PP has posted a link about Australian law!
If the charge is one of 'ABH' (rather than 'GBH' or 'GBH with intent'), see pages 11 to 14 here:
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
The use of a glass as a weapon almost certainly pushes 'culpability' into the 'higher' classification, meaning that the offence can't be classed as 'Category 3'.
If the court was of the opinion that the wounds were at the lower end of the 'ABH' scale, then 'harm' might be regarded as falling into the 'lower' classification, making it a 'Category 2' offence. For such an offence the 'starting point' sentence is one of 26 weeks custody (possibly reduced by one third for an early guilty plea). A non-custodial sentence remains a possibility though but the earlier caution reduces the chances of it.
However the court might well take the view that "a couple of deep gashes" push 'harm' into the 'higher' classification, which (together with 'culpability' already being 'higher', due to the use of the glass) means that it will be seen as a 'Category 1' offence for which (unless there are very exceptional circumstances) a custodial sentence is automatic.
Based upon the limited amount of information available here, my GUESS is that it will be seen as 'Category 1' ABH, with the lowest available sentence of 12 month's imprisonment initially being in the mind of the judge but with that then being reduced to 8 months for an early guilty plea, meaning that the offender will actually spend 4 months 'banged up'.
The foregoing though is based upon my initial assumption that an 'ABH' conviction will be sought and obtained. If the wounds resulted in serious permanent facial scarring to the offender the CPS might well try for a 'GBH' conviction, elevated to 'GBH with intent' because of the use of the glass. Pages 3 to 6 from my link would then apply, with the absolute minimum sentence available to the court (unless there were very exceptional circumstances) being one of 2 years imprisonment. (i.e. 3 years less one third for an early guilty plea). In which case the minimum time that the offender would actually be 'banged up' for would be one year.