Green’s Dictionary of Slang

indecent exposure.

1974		G.F. Newman You Flash Bastard 37: He had a newspaper across his lap; a wolly desperate to keep his numbers up might have fitted him for flashing.
1988		J. Healy Grass Arena (1990) 102: Fuck getting nicked for pissing; charge you with indecent exposure. Sounds bad, looks bad, like you been flashing or something.
2006		posting at www.talkmagic.co.uk 4 July 🌐 Someone once told me I have a passing resemblance to a feller who had been done for flashing in our area.