'Section 18' is an extremely serious offence. It's one one step below 'attempted murder', and carries the same maximum penalty of life imprisonment.
Depending upon the circumstances, judges have to decide whether an offence falls into 'Category 1' (the most serious), 'Category 2', or 'Category 3'.
Even a Category 3 offence requires a judge to use a 'starting point' sentence of 4 years imprisonment, with an actual sentence of 3 to 5 years. (However the sentence passed can then be reduced by one third for an early guilty plea).
So the lowest sentence which could be passed (other than in extremely unusual circumstances) would be one of 2 years imprisonment (i.e. 3 years, less one third). Normally half of that time would actually be spent 'inside'. So you can expect your mate to be 'banged up' for AT LEAST one year.
If the judge decides that the offence falls into Category 2, the starting point sentence (before knocking a third off for an early guilty plea) is one of 6 years, with a range of 5 to 9 years being available.
Category 1 has a starting point of 12 years, with a range of 9 to 16 years available.
It's unlikely that 'Category 1' would apply but the use of a knife might take the offence into 'Category 2'. In reality, you mate will probably be 'inside' for between one and two years.
Chris