Green’s Dictionary of Slang

Her Majesty’s carriage n.

a prison van.

1866		Wild Boys of London I 23/2: He was removed to a cell to wait the arrival of Her Majesty’s omnibus, which is painted black and made without windows.
1890–1904		Farmer & Henley Sl. and Its Analogues III 222/1: Her Majesty’s Carriage. A prison van, the King’s ’bus.
1902		H. Baumann Londinismen (2nd edn) 76: Her Majesty’s carriage.