To get the 'attempted GBH with intent (Section 18)' charge to stick the CPS would have be able to show that the offender intended to cause far greater harm than he actually did. (See here:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#b16 ). That could prove tricky unless, for example, he shouted "I'm going to kill you" or the CCTV evidence clearly shows, again for example, that he was aiming the knife at the victim's throat.
If the CPS could convince a jury that such an offence had been committed, the absolute minimum sentence a judge could pass (unless he considered that there were very exceptional circumstances) would be one of 3 year's imprisonment. Such a sentence can't be suspended.
However, based upon your account, I find it hard to see how the CPS could prove their case. It would seem far more likley that either the charge would be dropped to 'ABH' or that a jury would find him 'not guilty of attempted GBH with intent but guilty of ABH'.
ABH with a weapon (but carried out with a single blow, in the heat of the moment) might well be classed as a 'Category 2' offence. The 'starting point' sentence for such an offence, from which a judge must work up or down, is one of 6 month's imprisonment. However for a first time offender, with strong family ties, it's probably more likely that a non-custodial sentence (such as a Community Order, requiring unpaid work) would be passed (and/or a suspended prison sentence).
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf