A "SOCO" is a Scenes of Crime Officer, a UK law enforcement evidence gatherer.
Deleted scene: Mark Latimer (Andrew Buchan) shows up at the police station. He is angry with Ellie Miller (Olivia Colman) and Alec Hardy (David Tennant) for releasing information about the murder without consulting the family first.
Vicky McClure (Karen White) & Will Mellor (Steve Connelly) also worked together on two episodes of Line of Duty (2012) as Kate Fleming & Rod respectively.
Olivia Colman (Ellie Miller) & David Bradley (Jack Marshall) also worked together on Hot Fuzz (2007) as PC Doris Thatcher & Arthur Webley respectively.