If you've been bailed "upon suspicion of committing common assault, contrary to Section 39 of the Criminal Justice Act", that implies that the police anticipate that the Crown Prosecution Service will recommend that you be charged according. (i.e. with 'common assault' rather than with 'ABH' or 'GBH'). That would be consistent with your account of the victim's injuries. See here for the difference between the various charges:
http://www.cps.gov.uk/legal/section5/chapter_c .html
As that site indicates, 'common assault' is a 'summary only' offence. (There can be certain exceptions, but they won't apply in your case). That means that the case can only be heard before a magistrates' court (and not transferred to the Crown court). The maximum penalty is 6 months imprisonment (plus a fine of up to �5000).
In practice, the vast majority of first-time offenders convicted of a 'summary only' charge are not sent to prison. However, the court will be aware that someone else might have acted in a similar way to you, but with very different consequences. (If the impact of the bottle had fractured the guy's skull or the bottle had broken, severing an artery, you'd probably be facing a sentence of 3 tor 5 years, even for a first time offence, for 'GBH with intent')). So your actions will be regarded very seriously. (Get a good solicitor and apologise profusely. Don't try to justify your actions by relating the events you've described here, because this will only serve to convince the magistrates that you've got problems with alcohol and/or anger management. That would simply make things worse).
The most likely sentence is probably a non-custodial one, of around 100 to 200 hours unpaid work, plus a fine. (The size of the fine will be partly determined by your means but it might be around �300 + �100 costs + �15 victim surcha