It's never been clear in the UK (and indeed US) whether courts have allowed NDAs to trump criminal investigations.
In legal theory la-la land, it should not be possible to form a contract around an illegal action. Yet somehow, in the name of business, it appears to be allowed to tell the police to fuck off if you have signed an NDA.
As the recent MI5 lying to court shows that much like our airports, the legal system definitely has tow tracks. "Them" and "us".