From the CPS charging guidelines:
“The distinction between charges under section 18 and section 20 is one of intent. The gravity of the injury resulting is not the determining factor, although it may provide some evidence of intent.”
However, this is all academic. A guilty plea has been accepted to S20 and the CPS will not reconsider the charge.
You wish for a maximum sentence (which is five years) for the S20 offence. Page 8 of this document gives the details.
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
Accepting that this offence falls into “Category 1” (Greater harm and higher culpability) you will see that the suggested range is two and a half to four years. Assuming something approaching four years is deemed appropriate (this is by no means certain) you should remember that the offender MUST be given the maximum of one third discount for his guilty plea (which he entered at the earliest opportunity at the Magistrates’ Court). So you are probably looking at a little over two and a half years at most. Also (sorry to continually bear bad news) that the offender will serve only half of this time in custody with the rest being spent out on licence. Also (!) bear in mind that he will almost certainly qualify for early release under the Home Detention Curfew arrangement. This will be a further 90 days off. All in all he is unlikely to spend much more than a year inside and it may be less than that if the original sentence is less than four years.