Do they, for example, have a right to decrypt emails for the purposes of data mining? I'd have thought they needed a warrant and/or reasonable suspicion before

Do they, for example, have a right to decrypt emails for the purposes of data mining? I'd have thought they needed a warrant and/or reasonable suspicion before they're allowed to just mine anyone and everyone's private communications. Are UK police, for example, allowed to run fingerprint and facial matching algos against all image-based content uploaded to Facebook/YouTube/Instagram/whatever? I thought dragnet policing was considered to infringe the right to a private and family life, say? In short the police right to investigate is curtailed, and controlled by the courts and a warrant process, at least.