Answer
The Independent Custody Visiting Association (ICVA) were formerly known as lay visitors. To become an Independent Custody Visitor (ICV) you need to apply to your local Police and Crime Commissioner. You need to be aged over 18 and not working for the police service or in the criminal justice system or in any role where a conflict of interest may arise i.e., a solicitor. You also need a good command of the English language (all paperwork in cell areas is written in English). If you are successful, you will be given the relevant training for the role. ICVs are volunteers but do get reimbursed for their expenses.
The principal role of the ICVA is to visit detained persons in police custody and speak to them about the treatment they are receiving from the police and to check that they have had all their rights and entitlements. The detained person is under no obligation to speak to ICVs, it is purely a personal choice. The ICV can also check that person's custody record with their permission.
As they have a general responsibility to check the police are doing their work properly, they can do a check of all areas of the custody suite, ask questions and look at the records relating to the activities in the suite.
For more details on the role of the ICVA and how to apply please see the links in Related Information.