I think that the law is actually phrased that it's an offence to be holding your phone whilst in the driving seat of the car if the engine is on.
I do think that it's actually incredibly unlikely that if you had pulled over with your handbrake on anybody would like to try their luck in front of a magistrate by insisting on the letter of the law.
But I think it's phrased that way to stop those who'd think that they could make good use of the time when they're stopped at traffic lights.
You're 4 times more likely to have an accident using a mobile phone
This guy crashed and killed another driver parked in her car whilst he was using a mobile phone
http://news.bbc.co.uk/1/hi/england/beds/bucks/ herts/6263543.stm
He got 4 years