There appears to be a great deal of information about the assault missing from your post. The staff of the Crown Prosecution Service aren't idiots and they wouldn't decide upon a 'Section 18' charge unless they believed that there was sufficient evidence to obtain a conviction upon that charge.
The CPS has produced guidelines for its staff, indicating the appropriate type of charge for the different types of assault. You can be confident that the person who decided 'GBH with intent' to be the appropriate charge is fully familiar with the contents of this document:
http://www.cps.gov.uk...s_against_the_person/
Click on each of the relevant links to see the differences between 'common assault', 'ABH', 'GBH' and 'GBH with intent'.
Nobody here can advise you as to the likelihood of the charge being dropped to a lower level unless we're provided with full details of the victim's injuries and the detailed circumstances of the assault (including, for example, whether a weapon was used)
Chris