The judge (or magistrates) will have to refer to this document:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Sentencing guidelines for ABH appear on page 17. As printed there, they refer to first time offenders convicted after a trial. An early guilty plea can see the sentence reduced by up to a third.
You'll see that many ABH offences can lead to quite lengthy prison sentences but you'll also see that non-custodial sentences are still possible. With a good barrister (or solicitor if the case doesn't reach the Crown court) you stand quite a good chance of avoiding custody but nobody can give you any guarantees.
When you go back to the police station you might simply find that they're not ready to deal with the matter. If so, you'll just be re-bailed to another date. (That could happen several times). The police could also question you further but that's probably unlikely. Eventually (possibly after being re-bailed several times), you'll be charged and bailed to appear before a magistrates court. Don't be at all surprised if nothing happens on that date and you're simply bailed to a new date. At some stage the magistrates will decide whether to accept or decline jurisdiction. If they decline, there will be several more procedural hearings before the Crown court. (If the case does go to Crown court, expect it to be around 9 months, or possibly longer, before you're actually sentenced).