The definitions of the various kinds of assault charges are given in this document from the Crown Prosecution Service:
http://www.cps.gov.uk...s_against_the_person/
From that, you'll see that the appropriate charge is 'causing grievous bodily harm with intent'.
The table on page 13 of this document shows the sentences which judges are required to pass (other than in very exceptional circumstances, which the judge must explain in open court):
http://www.cps.gov.uk...s_against_the_person/
It should be noted that the sentences in that table refer to first-time offenders convicted after a trial. (Previous convictions, especially for violence, can push the sentencing range higher). An early guilty plea can see a sentence cut by up to one third.
It should also be noted that kicking someone in the head counts as the 'use of a weapon' and that committing an assault as part of a group is an 'agrravating factor'.
If, based upon the medical evidence, the court is of the opinion that your injuries were 'very serious' (but not life threatening) a sentence of around 5 years seems likely. If the court decides that your injuries fell within the 'life threatening' category, a sentence of around 8 years might be more likely. (Offenders sentenced to more than 4 years imprisonment MAY be released at the halfway point of their sentence but, unlike those serving shorter sentences, they don't become entitled to automatic release until two thirds of the way through their sentence).
Chris