Ah, now I'm confused. You say initially that you have been charged with a S20, but your last post now suggests that you are on police bail.
Reading between the lines, I think you have been arrested on suspicion of s20 assault, IF you are charged then I would suggest that it wil be a 47 abh rather than gbh.
With little corroborating evidence its your word aginst his and I suspect that this will go to a CPS lawyer for advice before any charges (if at all are laid) - you may end up with a CPS decision to discontinue and NFA the matter.
The law allows you to use reasonable force to protect yourself, even allows a pre emptive strike if you think you are in danger.
I'd certain seek advice when you return and bare in mind that most of the prosecution evidence in this incident appears to be coming from what you say on interview.