'Wounding with intent' is a very serious charge. It's only one step down the scale from 'attempted murder', and it carries the same maximum penalty of life imprisonment.
When using a weapon (such as a glass) it makes a significant difference to the sentence as to whether the weapon 'came to hand at the scene' or whether it was 'acquired prior to the offence and carried to the scene with specific intent to injure the victim'.
From your account of things, it seems likely that the court will decide that the latter applies. If so, the guideline 'starting point' for sentencing is 8 years custody. Judges are given some discretion but, unless there are exceptional circumstances, they should impose a sentence in the range of 7 to 10 years.
'Operating in groups or gangs' is regarded as aggravating the level of the offence, so that the sentence might be pushed up to, say, 9 years.
However, those sentences apply to first-time offenders convicted after a trial. An early guilty plea will result in a significant reduction in the sentence. My guess is that the offender will be sentenced to around 6 years imprisonment.
Source:
Page 13, here:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Chris