Yes, rojash is quite right. It is a "summary" offence for which the maximum penalty is six months custody.To support a prosecution the officer must have been acting lawfully in the course of his/her duty.
However, it is only used to prosecute assaults against police offficers which would otherwise be “Common Assault” if the victim was a civilian. More serious assaults against police are charged as Actual Bodily Harm, Grievous Bodily Harm or Grievous Bodily Harm with intent. There are no “enhanced” versions of these offences where the victim is a police officer (e.g. there is no “ABH on a police officer”). However, the fact that the victim was a police officer would seriously aggravate such an offence.