Crossed posts!
Get yourself a solicitor and, in the first instance, ask him/her to review the evidence against you in respect of the decision to charge you with GBH rather than ABH.
From the Crown Prosecution Service 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".
If those level of injuries weren't sustained by the victim, then the charge should only be ABH, not GBH. (Solicitors or barristers can often successfully argue such a case before the matter comes to court, so that you'd only face the lower charge).