Largely to back up TTT's post . . .
Much of motoring law relates only to'general'offences, rather than to 'specific' ones.
For example, there's no
specific law that says you can't drive on the right-hand-side of the road on a two-way street. If the police see you doing it they have to decide (along with the CPS) whether a charge of 'driving without due care and attention', 'driving without due consideration for other road users' or 'dangerous driving' is appropriate. (i.e. they can't actually charge you with "driving on the wrong side of the road" because there's no such offence). Consequently the milkman who makes his deliveries by driving his milk float down the right-hand-side of a deserted and well-lit road at 5am isn't breaking any laws (because his actions aren't 'dangerous', 'inconsiderate' or 'inattentive').
Prior to the introduction of the specific law prohibiting the use of hand-held mobile phones while driving, you could still be prosecuted for doing so if the police could show that you were therefore 'driving without due care and attention'. The new legislation simply provided clear rules where there was previously a somewhat grey area.
Similarly, the police (and CPS) have to decide whether other actions can be classed as 'driving without due care and attention'. There have been well-reported cases where drivers have been convicted of that offence for eating a Mars bar, or drinking from a can of Coke, while driving.
The CPS's own website also provides examples of some of the situations where courts have ruled actions to be illegal. They include "tuning a car radio, when the driver was avoidably distracted by this action" and "selecting and lighting a cigarette or similar when the driver was avoidably distracted by that use":
http://www.cps.gov.uk/legal/p_to_r/road_traffic_offences_guidance_on_prosecuting_cases_of_bad_driving/#a30
So fiddling with the controls of a radio, iPod or SatNav MAY result in a conviction IF the police/CPS can show that it means that the driving is failing to exercise due care and attention (but, unlike using a hand-held mobile phone, there is no specific legislation prohibiting such actions).