To cross the custody threshold Common Assault must fall into the sentencing guidelines’ “Category 1” (that is, greater harm and higher culpability). The greater harm aspect is certainly present. Details are on page 213 of this document:
http://sentencingcouncil.judiciary.gov.uk/docs/MCSG_Update9_October_2012.pdf
This was a sustained and repeated attack on a (comparatively) vulnerable victim. Higher culpability could also be suggested bearing in mind the assailant’s position of authority over the victim at the time. Even if it is accepted that the offence falls into category one custody is not an automatic option. The starting point for such an offence is a low level community order and considerably aggravating features must be present (over and above those already considered when determining which category the offence falls into) to take the sentence beyond a high level community order to custody. Also, offsetting the aggravating features was the fact that the PC suffered considerable provocation and (one assumes) he was of previous good character. In fact the sentence (150 hours unpaid work) is on the cusp between a medium and high level community order (see page 160 of the same document) and I believe that the District Judge got it about right.
People do not seem to understand that sentencing guidelines have made it very difficult for Magistrates and District Judges to impose custodial sentences for matters of violence in England and Wales. Very few people, especially first time offenders, go to prison for Common Assault and not that many are incarcerated for the more serious offence of Actual Bodily Harm. When considering the prison population in the UK it is often said that we are too quick to send people into custody when in fact the reverse is true. In comparable countries (e.g. Spain, France, Denmark, The Netherlands) matters of low-level violence almost always end up with a custodial sentence whereas in the UK the rarely do. The reason we have large numbers of people in prison is because we have large numbers of people who commit serious offences.