Petty crime
It's just pettiness by the police. They already have plenty to justify a warrant to get his telephone and internet records (if they haven't done so already) and so know all of his associates, people who may have tried to buy or borrow the guns, sell him ammunition etc. So there is zero chance of any sort of crime breakthrough as a result of accessing the phone itself.
However the police are measured by the Home Office statistically on 'solved crimes', and by pressing this charge we the taxpayer get to pay this bloke's accommodation for an extra six months and in return the the police get to claim one more "solved crime" as a success statistic.
I don't think that is good value for money.