Let's assume that this was their first offence. (Yes, I know that sounds unlikely!). Let's also assume that they try to get off by pleading not guilty. (Which seems to be far more likely). Those assumptions are because the relevant document, which judges refer to, provides guidance on sentencing first time offenders convicted after a trial. (Judges can then adjust the sentences downwards when the defendant pleads guilty or upwards if it's not a first offence).
Your description of events suggests that the offence falls into the category defined as "Pre-meditated wounding or
GBH involving the use of a weapon acquired prior to the offence and carried to the scene with specific intent to injure the victim (but not resulting in a life threatening injury or particularly grave injury)". That means that the judge should use a 'starting point' of 8 years custody, with a sentencing range available of between 7 and 10 years custody.
To read the sentencing guidelines for yourself, see page 13 of this document:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Chris