Green’s Dictionary of Slang

1. imprisonment within one’s cell, without association or exercise.

1974		Clement & La Frenais ‘New Faces, Old Hands’ Porridge [TV script] Midday, bang up [...] back to your cells.
1995		J. Cameron Vinnie Got Blown Away 69: Some nicks got twenty-three-hour bang-up.
2004		(con. 1990s) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 354: There are no bang-ups and, in fact, the cells are called ‘rooms’ and prisoners have their own key to their room.

2. the shutting of a cell door (at the end of the prison day).

1998		J. Hoskison Inside 8: After ‘bang up’ (an apt expression after hearing the door slam), we had ten minutes before ‘lights out’.
2000		J.J. Connolly Layer Cake 36: Most nights I was glad when bang-up came so I could be on my jack.
2014		D. MacShane Prison Diaries 51: Bang-up. About twenty minutes later the observation slit opens.