I totally agree but that is not read from the link you posted says, it gives the impression that if you were driving alone, a call came in & you pulled over to the side of the road, took the vehicle out of gear, put on the handbrake & switched off the engine, you were still 'driving' & therefore open to prosecution.
That's not how I read it. I would say that this bit gives what would be the general definition of driving.
Under existing case law, a person may be regarded as driving whilst the engine is running and the vehicle is stationary. This means that an individual stopped at a traffic light could be prosecuted for a mobile phone offence.
While this bit, including use of the word 'may', is only there to point out that there has been, in very specific circumstances, a case where the above definition was inadequate.
A person may be regarded as driving a stationary vehicle, even when the engine is not running.
I think that as it's using case law to interpret legislation, the first definition, in conjunction with the advice from the HC, ie 'safely parked', means that you would have to have parked somewhere or be doing something spectacularly and impossibly reckless for the second definition to come into play. Maybe door a cyclist while parked in a cycle lane and talking on your phone?