'Section 18' (GBH with intent) is an exceptionally serious charge, akin to 'attempted murder'. It carries the same maximum penalty of life imprisonment.
For a Section 18 GBH conviction (where the victim's injuries where not life threatening or particularly grave), involving the use of a weapon which came to hand at the scene (such as a glass), the 'starting point' sentence is 5 years imprisonment. Unless there are exceptional circumstances (which the judge must explain in open court) the judge can't pass a sentence of less than 4 years, or greater than 6 years..
However, those sentences refer to a first time offender convicted after a trial. Previous convictions (especially for violence) would push the sentencing range higher. An early guilty plea would see the sentence cut by up to a third.
We don't know all of the circumstances (such as any previous convictions you may have) but here's my estimate of the way the judge will pass sentence:
He'll start at 5 years. Then he'll take into account the provocation you faced, so he'll drop the sentence down to 4 years. He'll then check your previous record to see if he needs to move the sentence up again. (I'll assume that you've got no recent, or serious, convictions for violence). Then, assuming you've entered a guilty plea as early as possible, he'll knock a third off the sentence. So that comes to 2 years 8 months imprisonment.
Chris