Although you have been arrested under section 18, I think it highly unlikely that even if you are charged it would be under that section.
S18, GBH assault with intent (to injured) is as serious an assault as one can get. The lesser offences are section 20 GBH, section 47 ABH, and section 39 (common assault).
The next most serious offence to section 18 is attempted murder!
S18 is not commonly prosecuted because it’s the kind of offence when someone is extremely badly injured and have been severely beaten and the person committing the beating, intended to severely beat them but didn’t intend them to die.
Your solicitor will advise you but there is certainly a plea bargain to be done here of a much lesser offence (based upon your version of events). There has certainly been an assault and it’s unlikely that the defence of self-defence would work because you were not being attacked. There are mitigating circumstances but not a defence.
The definitive sentencing guidelines are here
If you go to section 18 on page 4 onwards, you will see the aggravating factors and culpability factors and the sentencing range.
If you then go forward to section 47 on page 12 onwards, (you can look at all the other sections of course) you will see the sentencing range for section 47.
I think it more likely that if you are charged it would be under section 47 or even 39, depending on the nature of the injuries. Based upon your account, I think there is very little chance of them securing section 18 and I think it’s probably something that they simply said at the time without knowing the extent of the injuries. It’s not uncommon if there is a lot of blood and they cannot at that stage see the injury
I’m not certain whether that answers the question for you or not but I am happy to answer any specific points arising from this
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