Green’s Dictionary of Slang

(UK, London school) a cry of warning.

1906		E. Pugh Spoilers 102: ‘Billo!’ exclaimed the boy hoarsely, ‘or there’ll be a rare ole chermozzle.’.
1938		F.D. Sharpe Sharpe of the Flying Squad 329: below (pronounced Bill-O) : Cry of warning meaning ‘Look out!’.
1938		J. Curtis They Drive by Night 214: ‘Billo’. A couple of youths came up to the top deck and sat behind Shorty. One of them looked at Molly with a hunter’s eye.
1977		D. Powis Signs of Crime 173: Bill oh! A shouted warning, ‘Look Out!’ (presumably from ‘Bill’, the police). Pronounced ‘below’.