The type of injuries which can result in a successful prosecution for GBH are illustrated on the Crown Prosecution Service website:
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#Unlawful_wounding
(See paragraph 55. If you scroll up to paragraph 46, you can see the type of injuries categorised as ABH, which better seems to fit your description. Scroll down to paragraph 65 to see the type of actions which take a GBH charge into the 'Section 18' category).
Like other contributors, I fail to see how the injuries you've described can fall into Section 18. However, if the defendant is found (or pleads) guilty, the judge is obliged to refer to this document:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
He (or she) can only go outside of the guidelines in that document under exceptional circumstance (which the judge must explain in open court). It's the table on page 13 which you should refer to, but you need to know that those sentences apply to first time offenders convicted after a trial. Previous convictions for violence will push the sentence higher. An early guilty plea might see the sentence reduced by up to one third. A sentence of around
5 years imprisonment would seem to be likely.
Chris