Actually the Information Comissioner has explicitly published his opinion on the personal use of dashcams to report "poor driving". Basically if the dashcam is personally owned and used for personal domestic purposes then there is no need for the owner to register with the Information Commissioner - https://www.inforigh...echnology-cctv/
However ... the news report doesn't tell us that the dashcam was privately owned, installed for purposes other than the recording of bad driving, mounted in a privately owned vehicle, which was not at the time being used for business purposes. If any of those four is not true then it would be possible that the owner of the dashcam needed to be registered with the Information Commissioner for the purpose, and display appropriate warnings visible from outside the vehicle, or was otherwise in breach of the DPA.
Personally If I was "the Police" I'd be very wary of accepting this type of evidence without assurance that the images were gathered lawfully. If I were the chap who has just been penalised I'd be very tempted to report the matter to the Information Commissioner.