Green’s Dictionary of Slang

(UK prison) the first landing of cells in a prison block; thus twos; threes; fours, etc.

1973		T. Lewis Billy Rags [ebook] ‘What about the lads down on the Twos?’ said Ray. ‘Are they in?’ The Twos were well pleased it wasn’t on their plate. You could tell.
1977		G.F. Newman A Prisoner’s Tale 47: ‘Come up to the threes,’ – there Collins held sway. [Ibid.] 49: ‘You ought to talk to Tony Scuffham down on the ones,’ Micky Dunkerton said.
2000		J.J. Connolly Layer Cake 148: Trevor’s party piece was throwin screws over the landing from the threes.
2020		G. Krauze What They Was 257: I get taken to the top balcony, known as the threes and the screw unlocks a door.