'Section 18' is an extremely serious charge. It's only one step below 'attempted murder and carries the same maximum sentence of life imprisonment.
The 'starting point' sentence (from which a judge can move the sentence up or down) for the least serious of 'Section 18' offences is 4 years imprisonment. Unless there are extremely exceptional circumstances (which the judge must explain in open court) the actual sentence should be between 3 and 5 years.
However kicking someone, while wearing shoes, can be regarded as 'using a weapon which came to hand at the scene'. That pushes the 'starting point' up to 5 years, with the available range being 4 to 6 years.
Those sentences refer to a first-time (adult) offender, convicted after a trial. Previous convictions can push the sentences higher but a guilty plea, at the earliest possible opportunity, can see the sentence cut by one third.
So the absolute MINIMUM sentence would normally be 2 years imprisonment (of which half would normally be spent 'inside').
Chris