Unless there are extremely exceptional circumstances (which the judge must explain in open court) judges are obliged to sentence first-time offenders, found guilty of a Section 18 offence after a trial, in accordance with the provisions of the table on page 13 of this document:
http://www.sentencing...inst-the%20person.pdf
That means that the minimum sentence would normally be one of 3 years imprisonment. (However if the court takes the view that premeditation was involved - which seems likely from your description of the prosecution's case - the normal minimum sentence would be one of 4 years). It should be noted that an early guilty plea can see a sentence cut by up to one third, and that the actual time spent 'inside' is usually only half of the sentence passed by the court.
A non-custodial sentence remains theoretically possible but the chances of such a sentence being passed for a Section 18 offence are virtually non-existent. (I've no statistics to hand but I'd be unsurprised to read that only one person in several thousand escapes prison).
Chris