The extent public authorities in the UK are employing private investigators for surveillance has been revealed using the Freedom of Information Act. 

Big Brother Watch report: 

Our latest report highlights the growing use of private investigators by local and public authorities, particularly the number of times they are used without RIPA authorisation.

The law in the UK, particularly the Police and Criminal Evidence Act 1984, is broadly drawn to allow evidence to be introduced in court that in other jurisdictions would not be deemed admissible. Contrasted with the fruit of the poisonous tree provisions in the US, and broader protection offered by the Fourth Amendment, UK law risks failing to join up the evidential admissibility process and the regulation of surveillance.

You can read their full report, see their data and release here. 

I am a journalist and author. I am a journalist at the UK edition of WIRED magazine. In 2015, my first book Freedom of Information: A Practical Guide for UK Journalists, was published. My second book Reed Hastings: Building Netflix, was published in March 2020. I created FOI Directory in 2012 and have maintained it in my spare since ever since.