The Government Says

Guidance: Prisoner marriage and civil partnership registration policy framework

Guidance: Prisoner marriage and civil partnership registration policy framework
Local copy of prisoner-marriage-civil-partnership-registration-policy-framework.pdf

https://www.gov.uk/government/publications/prisoner-marriage-and-civil-partnership-registration-policy-framework

seen at 02:52, 12 November in Ministry of Justice - Activity on GOV.UK.