There are very few laws which say exactly what you can or can't do while driving a motor vehicle. For example, there's no law that specifically states that you can't drive down the right-hand side of the road in a two-way street. If the police see you doing it they'll have to decide whether your actions contravene more general laws, such as those concerning driving without due care and attention or dangerous driving. (So the milkman who drives his float along the right-hand side of an otherwise deserted road, while delivering milk at 5am, almost certainly isn't breaking any laws).
Before the specific law prohibiting driving while using a hand-held mobile phone came into effect you could still be prosecuted if the police (and CPS) considered that you were therefore driving without due care and attention. Up until that time it would have been left up to a court to decide whether your use of a mobile phone was, or was not, illegal.
A similar situation now exists with regard to the wearing of headphones while driving. The police (and CPS) might well argue that an alert and responsible driver should be using his ears, as as well as his eyes, in order to drive to an acceptable standard. (i.e. he should be alert to the sounding of horns, the screeching of brakes or a parent shouting at a child who is about to run into the road). If so, they could commence a prosecution for driving without due care and attention but it would be left to a court to decide whether it was safe to convict under such circumstances.