I doubt that a GBH charge could be sustained. From the CPS website:
"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"
So an ABH charge would be more likely.
A vulnerable victim could see the case regarded as 'Category 2' (unless there were reasons for pushing it into 'Category 1', such as the attacker deliberately seeking out the offender to assault him). That means that the 'starting sentence', from which a court must work up or down, is one of 26 weeks imprisonment, with a maximum possible sentence of 51 weeks. However a non-custodial sentence may also be considered. See pages 11 to 14 here:
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf