First point:
The scenario you describe definitely falls into the 'GBH with intent' category. Read this document, from the Crown Prosecution's website, if you want to read up on the definitions. (If you want to keep the sentence relatively low you might need to modify the details of the crime accordingly):
http://www.cps.gov.uk...s_against_the_person/
Second point:
Here's the document which judges are obliged to refer to when considering sentence:
http://www.sentencing...inst-the%20person.pdf
Unless there were very exceptional circumstances, the judge would have to apply the sentences (modified as indicated below) shown in the table on page 13:
The modifications I've referred to come about because that table only refers to a first-time offender convicted after a trial. Previous convictions (particularly for violence) will push the sentencing range higher. An early guilty plea can see a reduction in sentence by up to one third.
Given that your scenario includes pre-meditation and the use of a weapon carried to the scene of the attack specifically for that purpose, together with the victim suffering particularly grave injuries, your first-time offender would (after an early guilty plea) probably be imprisoned for around 8 years. Perhaps you need to change something?
Chris