For a Section 18 offence the judge must examine both 'harm' and 'culpability', to determine whether they're to be regarded as 'low' or 'high'. If they're both 'low' the minimum sentence is 3 years imprisonment, with a maximum of 5 years. If only one is 'high' the range is 5 years to 9 years. If both are 'high' the range is 9 years to 16 years.
However those sentences apply to a first-time offender who is convicted after a trial. A previous conviction for violence will push the sentencing higher.
See pages 3 to 6 here to read exactly what the judge is obliged to read:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
A GBH charge (irrespective of whether it's under Section 18 or Section 20) will only be brought by the CPS when the level of injuries is extremely high. Here's the actual wording that CPS staff work to:
"Grievous bodily harm means really serious bodily harm. It is for the jury to decide whether the harm is really serious. However, examples of what would usually amount to really serious harm include:
injury resulting in permanent disability, loss of sensory function or visible disfigurement;
broken or displaced limbs or bones, including fractured skull, compound fractures, broken cheek bone, jaw, ribs, etc;
injuries which cause substantial loss of blood, usually necessitating a transfusion or result in lengthy treatment or incapacity;
serious psychiatric injury. As with assault occasioning actual bodily harm, appropriate expert evidence is essential to prove the injury"
Source:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a15
Given that such injuries must have occurred for a GBH charge to have been brought, I can think of no way that 'common assault' would be relevant in this case. The very best that could be hoped for would be for the charge/conviction to be reduced to ABH (if the injuries don't quite meet the criteria for 'GBH'). See pages 11 to 14, from my first link, for the relevant sentencing guidelines.