For most offences, the courts have to decide what Parliament intended or did not intend, based ostensibly on the wording of the legislation.
There is a general rule that courts must start with the presumption that Parliament did not intend to punish a given action unless the wording of the legislation convinces them otherwise, and that any ambiguity must be construed such that the action is not punished. No, really.
For drink driving, to pick a poor example, if a driver stop briefly, his level of intoxication does not materially change, so there would seem to be little material distinction as to whether he was actively driving the vehicle or sat in the back of the police car blowing into a tube when he is sat in the back of a police car blowing into a tube.
Did Parliament really intend that a driver who checks his phone briefly when he knows he will be sat at the traffic lights for a minute with the handbrake on should get 6 points - the same as a driver who is texting while driving past a school at 3.30? I would say not, but some would disagree.
However, Parliament's intention is not the only question (unless it is decided that Parliament agreed with me). If the device was being used to receive or share ideas or information, the qualified right to do so without interference by public authority under Art 10.1 ECHR is engaged. The question then becomes whether it is necessary and proportionate to restrict such rights in such circumstances to protect the rights or safety of others.
Not only does "it's the law" not in itself cut it, any justification for the restriction must be necessary and proportionate.
As regards texting while driving past a school, absent the most contrived emergency which would be covered by the statutory exceptions anyway, it would be very hard to argue that the restriction was not necessary or proportionate.
When sat at the traffic lights, there are arguments that might support Parliament intending to prohibit such behaviour (while still permitting a conference call if operated hands-free), but the hurdle for finding that such a restriction is necessary and proportionate is higher.
We hear that there have been numerous failed defences of not driving in mags' courts. I have not seen or heard the content of any such defences, or whether they sought to argue Art 10.1/HRA.
N.B. I would further argue that where such behaviour can potentially become problematic, but does not inevitably become so, there is no necessity for the specific restriction where a more general provision prohibiting the causing of the problem could readily be used. For example, if the argument is that a person briefly checking his phone might potentially then become entangled in some work issue that resulted in him not pulling away promptly and holding up those sat behind him at the lights - is there any good reason why the offence of driving without due care and attention/consideration if it did cause that?