There are several useful publications on Modern Slavery in the UK, below are listed a few.
The Home Office Statutory Guidance for England & Wales provides an overview of Modern Slavery, how to identify a potential victim, the responsibilities of various partner organisations, and the National Referral Mechanism process.
The second Modern Slavery Helpline report is available here.
The office of the Independent Anti-Slavery Commissioner published a Guide for councils on how they can respond to Modern Slavery.
The Gangmasters and Labour Abuse Authority have released a report detailing the Nature and Scale of Labour Exploitation in England and Wales.
The Modern Slavery Police Transformation Unit have released their 2017 annual report which reports on the progress the programme has made in its first year. The report can be accessed here.
The National Crime Agency releases National Referral Mechanism statistics quarterly and annually. The 2018 annual report can be found here.