There are various pieces of legislation dealing with the ability to monitor employees and employee rights in relation to being monitored:
Article 8 of the European Convention on Human Rights. This is contained in the Human Rights Act and so remains British law for now even after Brexit.
GDPR and the Data Protection Act
Telecommunications Regulations 2000
The Regulation of Investigatory Powers Act (RIPA) 2000 and Investigatory Powers Act 2016
The implied duty of trust and confidence inferred into employment contracts
The application of ‘fairness’ as applied when considering rights under the Employment Rights Act 1996
The Equality Act 2010