He's not contravened any specific legislation (such as that which prohibits the use of hand-held phones while driving) but most of motoring law relates to 'general' offences anyway. For example, there's no law which specifically states that it's illegal to drive on the 'wrong' side of the road. If the police see a driver doing so they have to decide whether his actions fall foul of legislation relating to 'driving without due care and attention' or 'dangerous driving'.
In the circumstances you describe the driver may well have been driving 'without due care and attention' but it might be hard for the police and CPS to obtain a conviction for it. (Cab drivers aren't subject to any driving laws which don't apply to other drivers).
It could have been worse. I took a taxi in Lille with a driver who watched a film on his DVD player (down by his knees) throughout the entire journey!