One rule for them and one rule for us
No, it isn't.
He was convicted of common assault. I am unclear whether this was classed by the court as a category 1 or category 2 offence. It is certainly higher harm, so it can't be category 3. However, to be category 1 there must also be higher culpability. It isn't clear whether any of the factors needed for this to be classed as category 1 were present.
If it was category 2, the starting point for sentencing was a medium level community order with the maximum being a high level community order. If it was category 1, the starting point was a high level community order with the maximum being 6 months. He pleaded guilty, which automatically reduces the sentence by one third. Any custodial sentence would almost certainly have been suspended. The fact there is a curfew requirement indicates he got a high level community order, which is in line with the sentencing guidelines.
It would be different if he had been convicted of ABH, GBH or attempted rape. Perhaps he should have been convicted of one of these offences, but the prosecution chose to accept a plea of guilty to common assault. As the sentencing guidelines stand, this was not a serious enough offence to warrant a jail sentence.
The fact he is a police officer made no difference to sentencing. Perhaps it should. Perhaps the courts should hand down higher sentences for police officers convicted of offences. I have some sympathy for that idea. But this is absolutely not a case of "one rule for them and one rule for us". In reality, the complaint is that it is currently the same rule for everyone.
I hope the police sack him.