You may know this already, but apparently the UK uses a National Referral Mechanism to collect and track cases of human trafficking which is organized by SOCA.
We were talking about needing better data - the Home office publishes its data from the NRM on it's website. I'm sending these out now and adding to the Google Drive.
SOCA (the Home Office agency) also provides another brief, synopsis of Human Trafficking in the UK. It provides us a lot of good data. (e.g. 2077 potential victims of human trafficking reported, 99 UK citizens identified as trafficked. etc)/
(Also in Google Drive under Research > General)