Green’s Dictionary of Slang

the police.

1856		H. Mayhew Great World of London I 6: Master Whelkey will answer perhaps, ‘But kool the esilop’ (look at the police).
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 23/2: Cool the esclop ... Look at the police.
1866		Graphic (London) 30 Jan. 23/1: ‘Slop’ is a corruption of the so-called backslang version of the word ‘police’ [...] so by this system police becomes ‘esclop’.
1870		Hotten Sl. Dict.
1926		Eve. Herald (Dublin) 8 Sept. 6/1: Slop is what is known as back-slang. Police spelt backwards is ‘ecilop’ which was shortened into ‘slop’.
1941		Post (Lanarks.) 10 Aug. 2/1: ‘Ecilop lortep snopuoc teg dir fo meht.’ [...] (Police ptrol coupns. Get rid of them). [...] This urgent message in backslang is alleged to have been shouted to a woman [etc.].
1953		J. Franklyn Cockney 297: A shout of, ‘Cool the esclop!’ (Look at the police[man]) drew attention to the advent of a constable.
1979		(con. 1920s) J. White ‘Campbell Bunk’ in History Workshop 26: Besides the occasional back-slang (police were 'slops' or 'ecilops') and the usually abbreviated rhyming slang (a Greek, for example, was a ‘bubble’ — bubble-and-squeak), were words and phrases from those traditions which helped make up the Bunk.
1977		D. Powis Signs of Crime 182: Esclop Policeman (London backslang).
1999		G. Seal Lingo 41: Thus the police became esclops, delo nammow is an old woman and a bad sort became a dob tros.