Green’s Dictionary of Slang

(UK Und.) to make a plan, e.g. for an escape from prison, a robbery (cf. plot up under plot n.).

1977		G.F. Newman A Prisoner’s Tale 49: ‘It does go the other way [...] I’ll have to plot one up, s’all.’ ‘Out of here? You got to be joking,’ Collins said.
2004		N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 13: We could ill afford a tug for [car] tax when we were tooled up and plotted on a bit of work.