Green’s Dictionary of Slang

Bushey Park n.²

a lark, a joke.

1859		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1883		N. Devon Jrnl 8 Feb. 7/2: [from The Echo] Call a flounder and dab with a tidy Charing Cross, and we’ll go for a Bushey Park along the frog and toad into the live eels.
1890		Sporting Times 22 Feb. 3/1: Don’t tell me to go to the Broad Walk in Bushey Park and shake some more down, if you happen to have heard it [i.e. a joke].
a.1909		press cutting in J. Ware Passing Eng. of the Victorian Era 57/2: Oh, it is a bushy park to see the Salvation souls toddling about arm-in-arm.
1935		A.J. Pollock Und. Speaks.
1953		J. Franklyn Cockney 293: Telling the potman to put it on the Cain and Able (table) [...] just for a Bushey Park (lark).
1960		J. Franklyn Dict. of Rhy. Sl.
1969		S.T. Kendall Up the Frog.