What you say about consent is correct in principle; someone who slapped someone else in a playful exchange, or two men who agree to box one another, could not be said to be guilty of assault; but the problem is one of degree.The law doesn't allow serious violence and injury whatever the parties think.
It's a matter for your barrister; s18 requires a Crown Court hearing, so a barrister would be engaged ; whether this 'consensual' wounding falls into the permissible category of consensual harm (don't raise your hopes).
If not, on the evidence available, then you'll have to plead guilty to something. The prosecution might accept a plea to the lesser offence of s20 wounding, rather than insisting on s18 wounding with intent, since it can be argued that you didn't intend the really serious harm, which intent that section requires or,less likely, that the injuries weren't sufficiently serious (it's the intent, not the consequences of the assault that matters).
Even s20 here may attract instant custody. On you version, you have some mitigation to put forward viz. the consensual arrangement, supported by medical evidence of injury to you. It's not an assault like an unprovoked attack , for example