To see the difference between 'ABH', 'GBH' and 'GBH with intent', read through paragraphs 45 to 67 here:
http://www.cps.gov.uk...he_person/#P189_14382
You might then have a better understanding of the chances of getting the charge reduced.
Unless there are genuinely exceptional circumstances (which a judge must explain in open court), judges must sentence within the guidelines shown in this document:
http://www.sentencing...person_accessible.pdf
See the tables on pages 13, 15 & 17.
However the sentences shown there refer to a fist-time offender who is convicted after a trial. Previous convictions (particularly for violence) can push the sentencing range higher, whereas an early guilty plea can see the sentence cut by one third.
If you're convicted under Section 18, the MINIMUM sentence would normally be one of TWO YEARS imprisonment (i.e. 3 years less one third for an early guilty plea) of which one year would actually be spent 'inside'.
Chris