While it is not illegal to be driving with snow on your roof, if you are caught - it's 3 points on the license and a £60 fine. It should really be illegal, because it's not exactly uncommon for the snow to slide along the roof and down the windscreen, nor is it uncommon for snow to fly off the roof of someone else's car and land on one behind it.
Meanwhile - re Datmoor:
I've just been reading that the fire in Cornwall is suspected to be Arson, and it's suspected that the Dartmoor fire is probably Arson too. I'm reading (on Twitter) that the fire crews are struggling to reach the fire because of it's location, and very icy roads. It is not clear if they have actually managed to reach it yet, but reports from people who can actually see it - are that it is still spreading, and spreading fast.
re Weather:
We are at -11.6C