Green’s Dictionary of Slang

(UK prison) the punishment cells; the maximum security cells.

1932		V.F. Nelson Prison Days and Nights 13: Paddy is powerless to punish me [...] he cannot send me to the ‘block’.
1982		G. Newbold Big Huey 46: They decided to keep us in maximum security for the night. They took us down to the block, the Separates Division.
1989		J. Morton Lowspeak 27: Block – solitary confinement in prison.
1998		J. Hoskison Inside 37: Probably get more painful, too, when they get him on the block.
2000		J.J. Connolly Layer Cake 154: As far as anyone knows he got moved that night down to the block.
2020		G. Krauze What They Was 263: I’ve only got a few weeks left [...] and I don’t want to spend them down block in solitary confinement.