NJ's summary overlooks the possibility of 'with intent' being added onto a charge of 'GBH', taking it up from 'Section 20' to 'Section 18'. Going to fetch a crowbar, and then waiting for his victim to arrive, would seem to be sufficient evidence for the CPS to seek a 'Section 18' conviction against the offender:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a16
This document explains the three different categories, '1' to '3' (with '1' being the most serious) that apply to both 'Section 18' (pages 3 to 6) and 'Section 20' (pages 7 to 10). The judge has to decide which of those three categories the offence falls into and then pass sentence accordingly (allowing a one third discount for an early guilty plea, where appropriate):
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
My guess would be that the offence would be classified as 'Section18' by the CPS (and subsequently by the court) and then as Category 2 by the judge. That would lead to a sentence of around 6 years imprisonment if the offender was found guilty after a trial or 4 years if he pleaded guilty from the outset.
If the CPS only sought a 'Section 20' conviction (or they sought a 'Section 18' one but the court only found the offender guilty under 'Section 20') it might well be classed as a 'Category 1'offence, leading to imprisonment for 3 years (after a trial) or 2 years (after a guilty plea).