You can read the advice that Crown Prosecution Service staff have to follow when determining the correct charge to lay before the courts, here:
http://www.cps.gov.uk/legal/l_to_o/offences_against_the_person/
As you state, a relatively minor cut doesn't qualify as 'GBH', so 'ABH' seems the far more likely charge.
The only caveat relates to the section in that document about
attempted GBH. As you'll see from the text there, any attempt to commit GBH should be charged under 'Section 18' ('GBH with intent'), rather than under 'Section 20' ('GBH') because 'intention' is inherent in the offence.
Your solicitor has read all the paperwork and his view that it will be an 'ABH' charge would seem to be far the most likely outcome. Other than with the most serious of offences, any first-time offender before a youth court MUST be given a referral order (which is akin to old-fashioned 'probation'). No other sentence is available to the court.