The level of injuries only determines whether or not an offence is classed as 'GBH' (rather than, say, 'ABH'). Entirely different factors dictate whether 'Section 20' or 'Section 18' is charged. Those factors are indicated here:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a16
If a weapon was used in the attack (which could, for example, include a booted foot), 'Section 18' would almost certainly be upheld.
The offence you refer to is likely to be classed as either 'Category 3' or 'Category 2'. (See Orderlimit's first link, above). If it's Category 3, the previous conviction (plus the new ABH) will push sentencing to the upper limit of 5 years custody, reduced by one third (for an early guilty plea), giving a sentence of 3 years 4 months (of which 1 year 8 months would actually be spent 'inside').
If it's Category 2, a sentence of 7 years, reduced by one third to 4 years 8 months (with 2 years 4 months 'inside') might be appropriate.
Sentences of over 2 years can't be suspended and there are many hundreds of insulin-dependent people in prison. Further, being drunk isn't regarded as a mitigating factor; courts often regard it as an
aggravating factor when considering sentencing.
Expect a sentence of around 4 years.
Chris