This looks promising (Mr Darcy as a detective investigating the Jack the Ripper murders). Synopsis from Radio Times:
Set during the aftermath of the Jack the Ripper murders in London’s East End, and currently filming in Dublin, the eight-part series from BBC America follows the Metropolitan Police H Division as they investigate the deaths.
A longer synopsis from Irish Film and Television News (the series is currently being shot in Ireland):
‘Ripper Street’ is set in and around Whitechapel in London’s East End in 1889, during the aftermath of the Jack The Ripper murders. The action centres on the notorious H Division, which is charged with keeping order in the chaotic streets of East London.
Jerome Flynn has been cast as detective sergeant Bennet Drake, Macfadyen as detective inspector Edmund Reid, and Adam Rothenberg as Captain Homer Jackson.