As Uncle Raymondo says, sect 20 is nowhere near as bad as sect 18. It is all abaout yours "mens rea". That is your guilty knowledge or intention to conflict a GBH.
If a GBH injury does exist (sorry to correct Ray, but the broken skin thing is a very, very broad definition and when it get to court will not be used, i.e you can have psychological GBH nowadays) it was a product of the whole situation.
That is, a jury may believe a fight was highly probable (whether self defence or not) i.e through provocation, but your actions were not intended to cause such a serious injury, it just happened.
You have not opted to go to crown court "to have your say", you have a say at magistrates. You have go to crown court to prove beyond all REASONABLE doubt your innocence. You will therefore need to convince a few jurors of your self defence defence. Remember self defence is just that a DEFENCE not an EXCUSE.
You have to promote yourself as in a position of danger. Your life or immediate wellbeing was threatened by a force capable of doing such damage. Even if the person injured had not assaulted you, YOUR actions was first NECESSARY and more importantly REASONABLE in the circumstances. There was not realistic chance of escape, it was them or me, I was in danger, I was scared, I had to defend myself. Learm "IMPACT FACTORS".
Impact factors will include numerous things and will PROVE you acted in Self defence:
You vrs 3 attackers
Your relative size
Your knowledge of fighting skills
Your sobriety vrs your attackers drink or drug levels.
Your stable mind vrs any mental illness of your attackers
Did they have weapons?
Were weopans near by? vases, tools, high heels etc. Did it cross your mind that they may have used them (hint hint yes it did)
Have you seen your attackers become violent before?