Perhaps 'froggy prosser', 'frog ponce', or one or another of those words, or some collocation thereof, would inspire the reaction described in the context you provided. The terms were chiefly derogatory, and common in popular and thieves' use in the 1890s. Although I could not readily locate attestation of the collocations in newspapers and books published in the 1890s, they are natural and likely collocations.
'Froggy', of course, was (and is) a common derogatory British term for the French, collectively or individually. Less likely is use with allusion to policemen, for which the appellation without the diminutive ('frog') would have been more common. Even so, the use of 'froggy' in a 'rough trade' setting in the 1890s would inevitably carry with it at least a hint of the 'policeman' sense along with the primary sense of 'despised Frenchman' and, more generally, 'despicable foreigner'.
froggy, n. and adj.2
2. Chiefly derogatory. Frequently with capital initial. A French person or (occasionally) French people collectively; (also) a French-speaker or person of French descent, esp. a French-Canadian. Also occasionally as a form of address. Cf. frog n.1 10.
OED provides four attestations of 'froggy' from the last three decades of the 1800s, including one from 1894, but these two uses from newspapers may be more telling:
A FRENCHMAN'S FREAK.
He Courted the Wife, not the Sister.
Froggy has £500 to Pay.
On Tuesday, in the Divorce Division of the Royal Courts of Justice, before Mr. Justice Butt and a common jury, the case of Lock v. Lock and Thommeret was commenced.
This was the petition of the husband, a clerk in the Inland Revenue, for a divorce by reason of his wife's misconduct with the co-respondent, Mr. Maurice Thommeret, a Frenchman, against whom damages were claimed. The co-respondent did not appear, but the wife pleaded condonation.
Worcestershire Chronicle, 01 March 1890, p 6 (paywalled).
A sharp look-out was kept for the foe; and hope ran high amongst the sailors at the thought of at last being able to have a "slap at Froggy."
Whitby Gazette, 14 February 1890, p 4 (paywalled).
The cross-referenced entry in OED, "frog n.1 10" is this:
10. Usually derogatory. Frequently with capital initial.
a. A French person or a person of French descent; occasionally as a form of address.
For evidence of 'frog' in the sense of 'policeman', see the entry in the 1889 A dictionary of slang, jargon & cant, by Barrere, Albert; Leland, Charles Godfrey:
and for both 'frog' and 'froggy', see the entry in the 1890 Slang and its analogues past and present, by Farmer, John Stephen, and Henley, William Ernest:
For 'prosser' and 'ponce', Farmer and Henley (1890, op. cit.) provide details of common use:
For other derogatory terms with similar meanings, see the list of synonyms provided for 'ponce' by Farmer and Henley.
Barrere and Leland (1889) present a much less detailed but here quite pertinent take on 'prosser':