You just need to think of the number of Police forces in the UK and there's not necessarily one for each county; some share. Let's say there's an average of six pilots who can be rostered at any one time for each unit (and that's a high estimate). It means there's not many positions available in the first place. Neither will it pay anywhere near as well as the airlines although I suspect job satisfaction is 100 greater!
Air Ambulance would be similar statistics but with the disadvantage that you have to clean the helicopter afterwards!!!!
Cheers
Whirls