Green’s Dictionary of Slang

1. (UK Und.) the Flying Squad.

1929		C.G. Gordon Crooks of the Und. 71: I am known to a good many of Scotland Yard’s famous Flying Squad [...] I was suddenly approached by two prominent members of the ‘squad’.
1938		F.D. Sharpe Sharpe of the Flying Squad 263: Jack, guy for your b--- life. The Squad are here and they’re pinching everybody.

2. attrib. use of sense 1.

1972

P. Fordham Inside the Und. 40: He was only to use it [i.e. a telephone] if he saw Squad cars.