(From a hazy memory) ..... In the early days, when there was only a handful of widely spaced Police ASU's around the country, the call-sign was 'India 99' = 999 where 'India' is the ninth letter of the alphabet. Over the years, as more Units became operational, different call-signs had to evolve by necessity. These call signs are now used for police comms air/ground contact purposes only.
When first formed the 'India 99' c/s, as used by the GMP, was used for both ATC and internal police comms, ( could also be India 97, 98 and 99 for the Met. which operated more helis). This changed some years later when, with the growth of Police Air Support Units across the UK, the Home Office decided to issue each Force area with its own number.
The ICAO designator for Police ASU in the UK is 'UKP xx'. ATC c/s 'Police xx". There is a list of ASU allocated numbers on the net somewhere.
The GMP ASU's aircraft have individual Force allocated ATC c/s. This takes the form of, 'Police 15(1), 15(2), 15(3)'. Where 15 is the Force (15 = GMP in this example) and (1) the individual a/c. If the Unit has one aircraft, then the ATC c/s should just be 'Police 24' (= Merseyside) or 'Police 22' (= Lancashire).