The absolute minimum sentence that a judge can pass upon someone convicted under Section 18 (other than in really exceptional circumstances, which don't seem to apply here) is one of 2 years imprisonment. (That's 3 years less one third for a guilty plea at the earliest opportunity). That means that you'd actually serve 1 year in prison, with the rest of the term on licence.
See pages 3 to 6 here:
http://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
(That's the actual document that judges are obliged to refer to).
Your best hope of avoiding prison is if your solicitor or barrister can get the charge lowered to Section 20, on the grounds that your 'intention' was not to cause serious bodily harm but simply to defend yourself. You might then escape with just a Community Order. (See pages 7 to 10 from my link above).