The court has to consider both 'harm' and 'culpability'. You've not actually mentioned the extent of your victim's injuries but, given that they were serious enough for the CPS to consider charging you with GBH, I must assume that that they are likely to push the 'harm' factor into the 'higher' category.
The use of a weapon would normally push 'culpability' into the 'higher' category as well. If both factors are at the higher level, the court must pass a custodial sentence of between 1 and 3 years imprisonment, with a 'starting point' sentence of 18 months. (The court could not pass a non-custodial sentence other than in genuinely exceptional circumstances).
However the use of excessive self-defence is a factor which could push the 'culpability' factor down to the 'lower' level. In which case (combined with the higher level of harm) the 'starting point' is still a custodial sentence (of 6 months) but the court can pass a non-custodial sentence instead. (The maximum sentence would be 51 weeks).
Those sentences are for first-time offenders who are convicted after a trial. They can be checked here:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf
(See pages 11 to 14)
A guilty plea (preferably at the earliest opportunity) can se sentencing reduced (by up to one third in the case of custodial sentences).
In my opinion you have absolutely no chance of avoiding conviction by pleading not guilty. If you enter such a plea you WILL be convicted and you WILL go to prison. If you enter a guilty plea but allow your solicitor/barrister to submit the mitigating circumstances, you PROBABLY WON'T go to prison. Speak to your solicitor/barrister a.s.a.p. to see if he/she agrees with me!
Chris