There are too many gaps in your account of events to state exactly why a Section 18 charge has been used. However, I can direct you to the document that the Crown Prosecution Service will have referred to when determining what they believe should be the appropriate charge:
See here (and, in particular, note the contents of paragraph 65):
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#P242_19963
I can also direct you to the document that judges have to refer to when passing sentence for Section 18 and Section 20 offences. The sentences shown relate to first time offenders convicted after a trial. Sentences can be reduced by up to a third for an early guilty plea. Otherwise, judges are bound by the contents of the document unless they believe that there are really exceptional circumstances (which they must explain, in open court, at the time of sentencing):
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
(For section 18, see the table on page 13. For section 20 refer to the table on page 15).
With the very limited information you've provided, my best guess is a sentence of 3 years imprisonment for a Section 18 conviction, or 18 months for Section 20. The chances of a non-custodial sentence would appear to be absolutely nil.
Chris