As far as the Crown Prosecution Service and the courts are concerned, when one person assaults another there are TWO 'victims' of the crime. One is the person who suffered the assault; the other is 'the law of the land'. Even if the person who sufffered the assault declines to provide evidence, the CPS policy is that they'll normally proceed with their prosecution since 'the law of the land' remains a 'victim'. (Additionally, of course, there is always a concern that the actual victim might have been coerced into not giving evidence).
So, while you could try indicating that you don't want the cae to go any further, it's likely that at least one of you will be prosecuted.
Chris