Journalist Richard Medhurst had his mobile phone seized. Did using a secure phone protect his data?

How can journalists protect confidential data and sources on their phones when crossing borders? Richard Medhurst, a well-known independent British journalist covering international affairs who has been highly critical of Israel, was arrested upon landing at London’s Heathrow Airport. He was the first reporter to be arrested by the British police under Section 12 of the Terrorism Act, which makes it an offence to invite support for a proscribed group.