Just for a bit of clarity (because the Press never make things like this very clear) it has always been illegal to use a mobile phone whilst driving (well, since the specific legislation was introduced). However, the law says specifically (and solely) that you must not use it for "interactive communication." Mr Ramsey Barreto had been convicted under that legislation in the Magistrates' Court. His argument that it was his son, not him who was using the phone was rejected. He appealed to the Crown Court and his lawyer drew attention to the fact that the phone was not being used for "interactive communication" but was being used to film the scene of a traffic accident. He cited an earlier Crown Court case where the defendant had been acquitted because he showed that he was using the phone to listen to music. Mr Barreto's conviction was quashed.
The prosecution, fearful of "floodgates" opening, took the matter to the High Court by way of a "Case Stated." The Justices at the High Court upheld the Crown Court's decision, saying that the legislation was quite clear in that it only prevented using a phone whilst driving if it was being used for "interactive communication" and that taking photographs or a film did not fall into that definition.
Since then, if a mobile phone charge is challenged, it has been incumbent on the prosecution to prove that the phone was being used for communication, hence the proposed change of law. Here's the judgement in Mr. Barreto's case:
https://www.judiciary.uk/wp-content/uploads/2019/07/19-07-31-DPP-v-Barreto-Ref.-CO2702019-Judgment.pdf