(2-part post):
You've posted 3 questions. I'll try to provide some answers in a single post.
Ultimately, it's for the courts to decide where the borderlines lie between ABH, GBH and GBH with intent. However, the Crown Prosecution Service try to get the charge right first time. To help their staff to do so, they publish 'charging standards' which take into account both statute law and case law. These are available online here:
http://www.cps.gov.uk/legal/section5/chapter_c .html
You'll see that 'intent' refers to 'intent to do some grievous bodily harm'. Further, a factor which might indicate such intent is "deliberate selection of a weapon or adaptation of an article to cause injury, such as breaking a glass before an attack". A good barrister may be able to successfully argue that, because the glass was only broken during the attack, and not prior to it, that 'with intent' should not apply but, as I've stated, it would be for the court to rule on this.