Unless there are extremely exceptional circumstances, judges are obliged to sentence 'GBH with intent' according to the table on page 13 of this document:
http://www.sentencing...inst-the%20person.pdf
However the sentences in that table apply to first-time offenders convicted after a trial. Previous convictions can push the sentencing higher (although I very much doubt that a caution for common assault would do so). An early guilty plea would see the sentence cut by one third. The actual time served in prison is usually half of the sentence.
Looking at that table, and reading your account of the events, it's hard to see a sentence of less than 2½ years imprisonment being passed (with half of that time being spent in prison). However I have seen one (and only one) case of GBH with intent, reported in the press, resulting in a non-custodial sentence. (It's extremely rare and requires that the judge is wholly convinced that there really are VERY exceptional circumstances).
Make an appointment to discuss things with your solicitor BEFORE you next attend the police station. Then insist that he/she arranges a 'conference' meeting, where you discuss things with both your solicitor AND your barrister, as soon as possible.
Chris