Regarding 'dropping the charges':
If you injure Fred Smith, the charge against you in court won't read "Fred Smith v Presh". It will read "Regina v Presh".
'Regina' (The Queen) effectively stands for 'the law of the land'. Even if Fred Smith was perfectly happy for you to injure him, the other 'victim' (i.e. 'the law of the land', a.k.a 'The Queen') wouldn't be. The CPS would still pursue the case.
The victim's injuries were at the lower end of 'GBH'. Read paragraphs 46 and 55, here, to see the difference between 'ABH' and 'GBH':
http://www.cps.gov.uk...he_person/#P189_14382
The sentencing guidelines for 'Section 20' GBH are on page 15 of this document:
http://sentencingcoun...person_accessible.pdf
You should note that the sentences there refer to a first time offender convicted after a trial. Previous offences (particularly for violence) can see sentencing pushed higher. An early guilty plea can see sentencing cut (by one third, in the case of a custodial sentence).
Chris