According to rule 229 of the Highway Code, before you set off: 

• you MUST be able to see, so clear all snow and ice from all your windows
• you MUST ensure that lights are clean and number plates are clearly visible and legible
• make sure the mirrors are clear and the windows are demisted thoroughly
• remove all snow that might fall off into the path of other road users
• check your planned route is clear of delays and that no further snowfalls or severe weather are predicted

If the snow on your roof falls onto the windscreen or backwards into the path of other road users, police say that drivers could be prosecuted for careless or inconsiderate driving if there is an accident.

However, South Yorkshire Police say that a text message doing the rounds stating that drivers will automatically receive fixed penalty notices just for having snow on their vehicles is a "hoax."

Full story - Mail Online 08/12/10