Alternatively, the defence could seek to show that the level of injuries sustained by the victim weren't sufficient for a 'GBH' charge (either under Section 20 or Section 18). The CPS website gives these as examples of the types of injuries which come under 'GBH':
* injury resulting in permanent disability or permanent loss of sensory function;
* injury which results in more than minor permanent, 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;
* injuries resulting in lengthy treatment or incapacity;
* psychiatric injury. As with assault occasioning actual bodily harm, appropriate expert evidence is essential to prove the injury.
http://www.cps.gov.uk/legal/section5/chapter_c .html#10
If the level of injuries sustained by the victim were lower than those described above, the defence could seek to get the charge dropped to 'ABH'.
'Self defence' would be extremely hard to prove. The defence would have to show both that the defendant could not escape from the situation and that he used the minimum amount of force required to defend himself.
Otherwise, unless the defendant can show that it's a case of mistaken identity, there's no defence available.
Chris