Even if you aren't holding it in your hand of looking away from the road, but are significantly distracted, you can be prosecuted under the "Driving without due care attention" law as you state.
Correct. However it is illegal to use a phone without being distracted dangerously, whilst using a satnav without being distracted dangerously is perfectly safe.
The Driving without due care and attention is a perfect catch all for dealing with bad driving behaviour. You can be done for eating crisps as easily as using a sat nav. The specific "using a mobile phone whilst behind the wheel of a car is 6 points on your license + a fine" is completely disproportionate in terms of what behaviours it implicitly approves (such as using sat nav safely whilst not moving) and the burden of proof (requiring the user to be "unsafe").
The reality is that policemen (of whom there are simply not enough) have discretion and a bit of common sense most of the time and use the tools at their disposal to punish people for driving badly whilst using their phone. This is good. I fundamentally disagree with singling out unsafe mobile phone use in law, however.
I also use google maps on my phone as a sat nav (audio only is plenty effective to get me places). Again, this is somehow dangerous and illegal to be using but the same behaviour with an actual sat nav, which in my experience is far more labourious to set a destination on, is fine.
I am, of course, aware of the law (I'm in the UK). However in this case the law is an ass. "Driving without due care and attention" properly covers every eventuality.
I wonder if this law will sit around ad infinitum whilst we ignore the ever more complicated computers sat on the centre console just as (if not more) distracting than a phone...