Green’s Dictionary of Slang

the workhouse, esp. the St Pancras workhouse.

1873		Sl. Dict. 304: ‘Pan,’ St. Pancras [workhouse]. ‘Pan’ and ‘Lump’ are now terms applied to all workhouses by tramps and costers.
1893		P.H. Emerson Signor Lippo 94: Next day all us kids were sent to the pan, and she got two months’ hard.