I believe that the phone has to be "fixed" into a holder that is in turn screwed/bolted to the car and with separate loudspeaker and microphone also you must not need to touch the buttons to answer or dial a call which to my mind makes most "kits" illegal. That said nobody seems to give a stuff about people using their phones when driving until they have an accident and then the rozzers ask them to blow down their alcohol measuring thingy and if that fails to prove an offence they check the driver's mobby records to see if they have been making or receiving calls in the period leading up to the accident.
That said:
Fixed rather than hand held seems to be fairly elastic depending on the circumstances and the policeman's attitude.
Personally I switch my mobby to voicemail when driving. I feel it is safer.