It has nothing to do with Chief Constables or the AA. The CPS would not prosecute a driver for operating a mobile phone in a cradle provided the driver wasn't distracted by texting or trawling the internet.
Personally, I think using a hands free mobile is just as distracting as holding on to your phone and I would never consider doing it. I also think that setting a sat nav when driving should be illegal as well. It is certainly frowned upon and the overhead traffic signs, here in Scotland, regularly tell you not to do it.
If I need to make a phone call or adjust my sat nav I find a suitable safe place to stop and do so there. And when I do I always switch off and remove the keys. It is illegal to use a handheld mobile phone, even if safely parked, it the car engine is running. Even if you are in a car park, if the public has access, it is an offence. As Michael Caine would say, "Not a lot of people know that".