The original charge will have been that agreed between the police and a local member of staff from the Crown Prosecution Service. It will since have been reviewed by senior CPS staff, taking into account all of the available evidence.
'Attempted murder' is an exceptionally hard charge to prove, since the prosecution needs to prove what was in the assailant's mind. The CPS staff have obviously come to the conclusion that it they would be unlikely to get a conviction for 'attempted murder' but that they will succeed with a 'wounding with intent' charge. Since both charges carry the same maximum penalty, of life imprisonment, they can probably see no point in wasting public money by continuing to seek a conviction for 'attempted murder'.
You can read the document which the CPS staff will have referred to when considering the case. This link takes you to the start of the 'wounding with intent' section. Scroll down to read the criteria the CPS uses for 'attempted murder'.
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/#P242_19963
You can also read the document that judges must refer to when passing sentence. It should be noted that the sentences in the tables refer to a first-time adult offender convicted after a trial. An early guilty plea can see the sentence cut by up to one third; previous conviction can push the sentencing range upwards. While the document does not directly refer to juvenile offenders, it does show that a conviction for 'wounding with intent' nearly always leads to a substantial custodial sentence:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Chris