The 2019 UK Annual Report on Modern Slavery https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/840059/Modern_Slavery_Report_2019.pdf sets out an assessment of trends in modern slavery in the UK. The Home Office also publishes quarterly statistics on potential victims being referred to the National Referral Mechanism. https://www.gov.uk/government/statistics/national-referral-mechanism-statistics-quarter-3-2019-july-to-september
In March 2020, the Office for National Statistics published a report entitled ‘Modern Slavery in the UK: March 2020’, https://www.ons.gov.uk/peoplepopulationandcommunity/crimeandjustice/articles/modernslaveryintheuk/march2020 which brought together data sources linked to modern slavery. The report noted that there is no definitive source of data or suitable method available to accurately quantify the number of potential victims of modern slavery in the UK. However, we are committed to improving our understanding of this complex and hidden crime. In July 2019, the Government announced a £10 million investment to create a new Policy and Evidence Centre for Modern Slavery and Human Rights. We will continue working with the Centre and other partners to strengthen the evidence base of the nature and scale of modern slavery in the UK.