So (before taking into account an early guilty plea) the judge should be looking at a minimum sentence of 7 years imprisonment. He might find that there are exceptional circumstance which can take that down to 6 years. Then he can knock a third off for an early guilty plea. That means that you might be facing a sentence of around
4 years imprisonment.
Your best chance of getting a lower sentence than that is to get the charge dropped to ABH. See paragraphs 46 and 55 of this (Crown Prosecution Service) document to see the difference between 'ABH' and 'GBH':
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#P189_14382
The sentencing range for the category of ABH defined as "pre-meditated assault involving the use of a weapon" is 2 to 4 years imprisonment. The element of provocation could see the judge start at the lower end of that range, and then knock a third off for an early guilty plea. That makes
16 months imprisonment
Chris