There are several useful publications on Modern Slavery in the UK, below are listed a few.
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 2017 annual report can be found here.