Green’s Dictionary of Slang

Bill, the n.

also Bill
[ety. unknown; ? abbr. Old Bill n. (1). There is a poss. semantic link to beak n.1 (1), but in reality it is unlikely]

[1960s+] the police; thus Bill from the Hill, officers serving at Notting Hill police station in London W11.

In compounds

bill shop (n.)

[1960s+] a police station.

bill-wagon (n.)

[2000s] a police car.