The offence is described (s42(D) of the Road Traffic Act 1988, as amended) as 'driving a motor vehicle while using a hand held mobile telephone or other hand held interactive communication device'.
That raises the interesting question of whether someone using an ordinary camera in these cirumstances commits an offence. Plainly they'd commit no offence under this section, as the camera would not be a phone or 'device' for this purpose. So does the fact that the camera is an adjunct to a phone make use of it an offence? Surely not, for the words 'or other hand held communication device' make it clear that it the phone itself has to be being used as such a device; it's the communication which is is the mischief to which the section, and any regulation made under it (there is a reference to construction and use regulations in the section), is directed.
This argument is supported by the magistrate's decision in a case against Jimmy Carr, obviously not binding authority, that using the phone merely as a dictaphone was not an offence.
The drivers could, perhaps, be prosecuted successfully for another offence, such as driving without due care and attention, or failing to keep proper control of a vehicle, but that depends on the circumstances.