The policy of the CPS would normally be that it would be 'in the public interest' to pursue a prosecution if there was a reasonable chance of conviction. (The attitude of the CPS is that every crime has TWO victims. One victim is the person who suffers directly from the offence, but the other 'victim' is 'the law of the land' or, if you prefer 'public order'). Even if the 'actual' victim is not interested in the offender being prosecuted, the interests of the 'law of the land'/'public order' require that a prosecution should still normally be undertaken.
The victim could be summoned to court as a witness and required to give evidence. Failure to attend, or to give evidence, could possibly result in his imprisonment. (Defendants have a right to silence. Witnesses don't).
If you're seeking to determine whether the offence should be classed as 'ABH', 'GBH', GBH with intent' or 'Attempted murder, see paragraphs 45 to 75 here:
http://www.cps.gov.uk...he_person/#P189_14382
Chris