The employee has the right to refuse to provide mobile phone records. However, if an allegation is serious enough and there is sufficient evidence for a case to answer the case can be reported to the BTP or ORR, who can gather the evidence forensically.
If the alleged event is serious enough and proven through a court the remifications could be more severe. For example using a mobile while driving a train, if it goes to court would more than likely result in a custodial sentence. Where if the same charge is heard at a company level although the burden of proof is different, the ramifications would be slightly less severe.
I would advise anyone who's management requests sight of records to provide them, redacted if required for privacy, in order to prove innocence.
If I GENUINELY suspected an employee of inappropriately using thier mobile device while carrying out SC duties and they didn't cooperate during an investigation process, I would report it to the ORR.