But why is it against the law to have the tv on in the dash while driving , but it is okay to glance over now and again to see where the Sat Nav is taking you

As far as I'm aware, it isn't specifically 'illegal' to have the TV on. The law demands that the drivers attention is limited to the road, and actions such as smoking a cigarette or tuning a radio have been deemed as 'careless driving' offences where such action has directly led to an incident. The same would obviously be said for watching a TV.  The older I get, the more I realise that people confuse wrinkles for wisdom