The actual document that judges must refer to is here:
http://sentencingcoun...ine_-_Crown_Court.pdf
See pages 8 to 10.
As your offence would seem to fall within 'higher culpability' (because of the kick to your victim's head) but 'lower harm' (because the injuries weren't life-threatening or the cause of severe permanent disability or disfigurement) it's likely that the judge would see it as a 'Category 2' offence. That means that it passes the 'custody threshold' test, with a sentence of around 12 months imprisonment being likely. (Based upon 18 months less a third because of your guilty plea).
However it's less likely that the circumstances push the sentence beyond the point at which it can be suspended. So a 12 month sentence, suspended for two years, with a Community Order and an order to pay compensation to the victim would seem to be the most likely outcome to me.
Even so, it would still be wise to pack a bag when you attend court for sentencing. (A practical point here: Keep the bag with you or, if you're not permitted to, hand it to a court official or security officer. If you give it to your barrister or a family member, they won't be able to get it to you if the worst happens and you're 'sent down').
Chris