ADH? Never heard of it!
I assume that you mean 'ABH' ('assault occasioning actual bodily harm').
Sentencing guidelines for ABH are on page 17 of this document:
http://www.sentencing...person_accessible.pdf
It should be noted that the court may well regard a kick to the head as the 'use of a weapon'. See Section 22 on page 6 for confirmation of that fact.
It should further be noted that the sentences shown there refer to first-time offenders, convicted after a trial. A guilty plea, at the earliest opportunity, can see a substantial reduction in the sentence (which is normally one third in the case of a custodial sentence).
Since the offence was not pre-meditated, the lowest sentencing band would seem to apply. The 'use of a weapon' (i.e. your shod foot) MIGHT push the sentence towards the maximum 26 weeks custody (say 24 weeks), which could then be reduced by one third, to 16 weeks, because of your guilty plea. You'd then serve half of that period (i.e. 8 weeks) in prison.
However it's probably far more likely that the court would take the view that the kick pushes the sentence above the 'starting point' of a high-level community order but your guilty plea take the sentence back to that point.
So, based upon your account of events, my guess is that you'll receive a high-level community order (i.e. 'probation' + perhaps 200 hours unpaid work).
Chris