The Employment Rights Act 1996 protects whistleblowers from detriment or dismissal where they make public interest disclosures, providing those disclosures meet certain conditions. Gagging clauses which seek to prevent workers from disclosing information are unenforceable insofar as they seek to prohibit whistleblowing.

The House of Commons has produced a briefing paper which provides “an overview of protection under the Employment Rights Act 1996 for workers who disclose information about malpractice at their workplaces.”
A link to the full briefing paper is available here:
http://researchbriefings.parliament.uk/ResearchBriefing/Summary/CBP-7442