//It is now specifically illegal. Following changes on 25 March 2022, motorists are breaking the law if they use a handheld mobile phone behind the wheel for any use.//
As Gromit points out, Zacs, it has been a specific offence since 2003. All went (relatively) well until 2017 when a certain Ramsey Barreto was caught using his phone to photograph an accident on the adjacent carriageway. He was prosecuted for mobile phone use but declined the offer of a fixed penalty. Instead he pleaded not guilty in the Magistrates' Court, saying that it was his son, sitting in the passenger seat, who had used the phone. The Magistrates rejected his defence so he appealed to the Crown Court, using the same basis. The Crown Court also rejected his defence but his barrister cited a recent case at Harrow Crown Court where the defendant was acquitted of using his phone to listen to music because it did not involve any external communication (as the statute requires). As a result of this, Mr Barreto was acquitted by the Crown Court but the DPP, fearful of floodgates opening, took the matter by way of a "case stated" to the High Court for clarification of the law. The High Court ruled against the DPP:
https://www.bailii.org/ew/cases/EWHC/Admin/2019/2044.html
That was in July 2019 and the ruling meant that, from then on, the prosecution had to prove that the phone was being used for "interactive communication" for a prosecution to succeed.
The government acted swiftly (!) and in March of this year an amendment was introduced to s110 of the Road Vehicle Construction & Use Regulations prohibiting virtually all use of mobile phones whilst driving. This is the change that you are referring to. There is, however, a specific exception to allow their use for making contactless payments at drive-through establishments.