It may be that all police aircraft in the UK are insured these days but there is no requirement for the police forces to insure through the commercial sector. That is why the Met Police were always more gung ho and did the job on the streets... they did not have insurance companies breathing down their necks.
Large forces like the Met have a history of not insuring, and the same happens across the world with large operators. The price of helicopters and other capital equipment may have changed the situation.
I am unsure where the Met are these days - individually the aircraft are expensive and a big slice of a budget if the craft are damaged - but there is ample evidence that earlier fleets were not insured. I assume that is why Lynx thought he was on good ground for saying that.
So, are they all now insured by Third parties?