The whole area of occupational language is a good one to study. Doctors, teachers, soldiers, call centre workers and firefighters are all likely to have their own occupational varieties, which themselves might be affected by the speech styles of the individuals in the job, or the geographical area that people work in. The police are no exception to this and their occupational variety is probably one of the more widely scrutinised forms because it often appears in transcript form in court, or (perhaps more glamorously) on TV dramas and even reality police shows.
So is this sort of occupational variety a slang or a form of jargon? To what extent might we call the police a "community of practice"? According to the definition given by Graeme Trousdale in his excellent new book, An Introduction to English Sociolinguistics, a community of practice is defined by three characteristics: mutual engagement, a jointly negotiated enterprise, and a shared repertoire. He uses the example of a school rock band to illustrate these characteristics.
Here, a number of individuals come together in face-to-face contact (mutual engagement) for a particular purpose, that is, to play music (jointly negotiated enterprise), often conversing using jargon common in discourse on popular music, such as riff, bridge, amp, bass guitar and so on (shared repertoire).
Is the same true for the police? I think so. And like other communities of practice, there is pressure from within to use language in the accepted way, meaning that members of the community of practice become institutionalised in the language of the organisation.
Quite where this leaves the firearms police involved in this case, I don't know. they seem to have developed their own discourse, which apparently consists of slotting in as many song titles as possible into their witness testimony...
If you're interested in the language of the police, this link to a forum about police slang/jargon might be of interest.
Also, the many different words used to describe the police can be found here.
edited to add:
While on the subject of communities of practice, this short article about the language used on message boards by divorced women discussing their relationships is quite a good one. There's the usual mix of acronyms and initialisms that we tend to see online (although I'm not sure LOL has been correctly defined here) along with a smattering of new ones like STBXH (Soon to be ex-husband) and OW (other woman).