Firstly, you don't appear to have described 'Grievous Bodily Harm'. The Crown Prosecution Service's own guidelines state;
"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"
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a15
So it's far more likely that you've described 'ABH' (if any offence at all was committed).
Secondly, even if the CPS could get 'GBH' to stick, there's probably sufficient doubt present to make adding on 'with intent' unlikely to stick:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/#a16
Thirdly, although the vast majority of people who post here claiming that there actions were 'self defence' haven't got a snowball's chance in hell of being acquitted, you genuinely seem to have a valid defence:
http://www.cps.gov.uk/legal/s_to_u/self_defence/#Reasonable_Force
However, to answer your actual question IF (and it's a very big 'IF'!) you were actually convicted of GBH with intent then, yes, you would definitely go to prison.
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
(That's the actual document that judges must refer to when sentencing. Pages 3 to 6 apply. If you were charged under Section 18, pleaded 'Not Guilty' and were then convicted, the minimum sentence that could be passed would be one of 3 years imprisonment, meaning 18 months of actually being 'banged up').