>>>i'm just astounded that it hasn't been flagged up as a separate charge
The vast majority of motoring law is founded upon 'general' legislation, rather than 'specific' rules. For example, there's no specific law that states that you can't drive on the 'wrong' side of the road. If the police see you doing so they must (together with the CPS and the courts) decide whether you're driving 'carelessly' or 'dangerously'. If neither applies then your actions aren't illegal. (So a milkman who drives his float along the right-hand-side of a road, while making deliveries on a well-lit and otherwise deserted road early in the morning, ISN'T breaking the law).
Legislators have decided that using a hand-held mobile phone ALWAYS results in a loss of attention from a driver, and therefore decided to make it a specific offence. However it might be hard for them to list every possible action that might also result in a loss of attention. They'd need to include reaching for a cigarette packet, taking a cigarette from a packet, lighting a cigarette, unwrapping a confectionery bar, consuming a confectionery bar, opening a can of drink, drinking from a can, etc, etc, etc. The list would be almost endless. Further, they'd need to decide whether tuning a radio manually to find a station would lead to a loss of appropriate attention, whereas simply pressing a 'scan' button might not.