It's most definitely the appropriate charge. If you want to read the difference between 'ABH' and 'GBH' see paragraphs 46 and 55 here:
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/
To see what adds 'with intent' onto a GBH charge, read paragraph 65.
This is the document which judges must refer to when sentencing. (They're only permitted to go outside of its guidelines under very exceptional circumstances, which the judge must explain in open court). The table on page 13 is the one you need to refer to. The sentences shown apply to first time offenders, convicted after a trial. (Previous offences can result in sentences higher than those shown in the table. An early guilty plea can result in a reduction of the sentence by up to one third):
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Since the victim's injuries seem to fall into the 'life-threatening or particularly grave' category, and assuming that the offence was not pre-meditated (and also that the offender has no previous convictions), the judge must work from a starting point of 8 years imprisonment. If there were mitigating circumstances, such as provocation, he's allowed to go down to 7 years. Knocking a third off for an early guilty plea results in a
minimum sentence of
4 years 8 months imprisonment.
Of course, we don't have all of the information available which the judge does, but 'around 5 years' seems to be a reasonable guess. (The maximum penalty for 'Section 18' is life imprisonment).
Chris