It would appear that rumours are that they could be fined for driving with snow on their roofs.
Checked with Road Policing Unit said that there is no specific legislation on driving with snow on the roof of a vehicle, however if it slips over the windscreen, or flies into the path of another car, it could leave the driver open to being penalised for driving without DUE CONSIDERATION, DANGEROUS DRIVING, NOT BEING IN PROPER CONTROL OF THE VEHICLE, NOT HAVING A FULL VIEW AHEAD AND WINDOW NOT BEING SUFICIENTLY CLEARED.
All which carries their own various penalties
“Any snow left on vehicle bodywork, such as the bonnet or roof, could slide onto lights or windows causing dangerous obstruction to vision of the driver or signals to other road users.
“Any large amount of snow could slide from a moving vehicle and onto a footpath or roadway possibly causing danger or injury to other road users or pedestrians.â€