To give an illustration, there are sentencing guidelines in England and Wales that give judges parameters in which they need to operate. The "starting point" for Grevious Bodily Harm causing permanent brain damage to the victim is 3 years' custody. To this aggravating and mitigating circumstances get applied (and headbutting is considered one of them since it's a weapon). Being drunk is not a mitigating circumstance - it is an aggravating one - but is often brought to the judges' attention to explain what happened, and also to make it clear there was no premeditation. Guilty pleas and previous good behaviour will reduce the sentence.

I actually think this is pathetic sentence since it's about the same level as an accountant defrauding their employer or a serious tax fraud.