Green’s Dictionary of Slang

a £1 note, thus a banknote.

1916		‘Taffrail Stand By! [ebook] [note] A ‘Bradbury’ is one of the new £1 notes. So called from the signature at the bottom.
1919		W.H. Downing Digger Dialects 13: brad — English pound note. (From ‘Bradbury, Secretary to the Treasury’).
1919		‘Sapper’ Mufti 154: He might continue the good work on other prospectuses, thereby pouching more Bradburys.
1924		‘Digit’ Confessions of a Twentieth Century Hobo 18: A ‘John Bradbury’ and I was on board.
1932		(con. WWI) L. Mann Flesh in Armour 266: ‘I gave the elder [boy] a quid. “Cripes, digger [...] it’s a Brad”’.
1945		‘Henry Green’ Loving (1978) 121: ‘Pounds?’ she said making her eyes big. ‘Lovely British Bradburys,’ he answered.
1956		R.T. Hopkins Banker Tells All 61: The first batch of ‘Bradburys’ were very poor notes.