The courts - give evidence as prosecutions witnesses; providing protection for vulnerable witnesses; holding defendants in custody and transport them to and from court.
The CPS - providing evidence for prosecution; charging offenders with CPS instructions.
HMPPS - police arrest prisoners who have been recalled to prison; as a result of Sarah's Law, the police also cooperate with the HMPPS in managing the list of child sex offenders living in their area.
Voluntary organisation - e.g. referring victims and witnesses of crime to Victim Support, women's refuges, the Witness Service etc.