ABH and GBH (without intent) are both 'either way' offences, which mean that they can be heard before a magistrates' court or referred up to the Crown Court. GBH with intent has to be referred to the Crown Court. In practice (unless you can get the charge dropped to ABH) it's likely that you'll eventually appear before the Crown Court. There will be several procedural hearings in the magistrates' court, and then yet more in the Crown Court, before anything substantial actually happens, which is likely to be in around a year's time. (Theoretically you could be remanded into custody at any stage but, in practice, you'll almost certainly be repeatedly bailed).
If you're convicted, the judge will refer to this document, which relates to sentences for first-time offenders convicted after a trial. (See page 13 for GBH with intent, page 15 for GBH and page 17 for ABH):
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
The element of provocation you faced would probably push your sentence to the bottom of the range shown. An early guilty plea would see a third deducted from the sentence. The actual time served in prison would normally be half of the sentence.
Don't wait until you have to return to the police station. Contact a solicitor immediately. He (or she) will then have studied all of the evidence before he/she accompanies you on your next visit.
Chris