The recent legislation against people using mobile phones when driving does not apply to CB radio.

The difference is that with a mobile phone, the conversation is fully two-way (known as duplex), whereas with CB and PMR446 etc, you have to take turns transmitting and then receiving (simplex) and this is not considered to be as distracting as a full conversation.

I have found a very informative page about the legislation Here if you want further details.