'Section 18' is an extremely serious charge. It's only one step below 'attempted murder' and carries the same maximum penalty of life imprisonment.
Judges are given sentencing guidelines which state the the lowest level of 'Section 18' offences should be sentenced with a 'starting point' of 4 years imprisonment. The judge then has to consider the relevant factors involved in the case, but the final sentence should be in the range of 3 to 5 years.
However, when there's an element of premeditation (such as paying the guy a visit accompanied by a few of your mates), the starting point goes up to 5 years, with the sentencing range being 4 to 6 years.
Those sentencing guidelines apply to adult first-time offenders, who're convicted after a trial. An early guilty plea can result in a reduction of the sentence by up to a third.
While it's hard to be precise, I'd guess that, in view of your youth, the court would consider passing a sentence of around 2 to 3 years youth custody. However, with a good barrister, there's still a reasonable chance that you might avoid being 'sent down'. (In which case you'd be looking at something like 200 hours of unpaid work, plus a fine).
Chris