A slight correction or comment on oh's correction,
He is right that JAR OPS does not apply BUT the powers that be (i.e. the CAA in the UK) generally apply the same standards across onto police operations with a short time lag.
So for example,
Police Ops over a congested area require a twin;
Police Ops at night require a twin (cos police observers are considered as pax, pax means "public transport" and pt means twins at night)
If you want to do UK police ops day only and over open non-hostile terrain, you could still use a single in the UK........... but there ain't much point because most police work involves people where they live.
I think that Buitenzorg's comments are a great summary of the difference between USA and the offshore bit of Europe, and why we got to where we are.