If they look at mine they will find over half of the stored numbers are for the Met police, Surrey police, Kent police, Sussex police, MOD and other such institutions and the calls would be very boring.
I was thinking of changing my identity in the same way that Sherlock Holmes did when he wanted to get a result when capturing the criminal under investigation ...even Dr. Watson was confounded and didn't have a clue when Holmes peeled off his mascara false eyelashes and woman's tights...
Irish comedian Dave Allen did a good sketch where the camara followed the top half of his body dressed as "Jack The Ripper" chasing after a young damsel, femininely attired in skirt n stockings n high-heels (the bottom half)...when the camera panned out to show Dave, he was dressed as Jack The Ripper top half and The damsel in distress he was chasing, the bottom half...a classic TV sketch.
"what happens under the official secrets act concerning the police and the extraction of sensitive data"
The Official Secrets act isn't involved here. Mobile phone data would be covered by the data protection rules if it could be used to personally identify an individual, and if it was used for any purpose other than as evidence in a criminal prosecution.
The Police position would no doubt be that the data obtained from phones where no prosecution is brought will be held securely, will not be passed to a third party, and will be destroyed after a period of time. To which my response would be to ask why it's considered necessary to retain it in the first place.
Probably held as securely as all the other evidence which seems to go missing all to often. They keep telling us their budgets are being cut too much so why the need to spend lots of money needlessly keeping all this info.