Whilst there is no specific offence relating to snow on a vehicle's roof and bonnet, it is advisable to remove any loose snow before commencing your journey. If it slips or is blown over the windscreen, or flies into the path of another car, it could cause a hazard for you or another road user and could leave you open to being penalised for a number of offences, such as 'driving without due consideration' or 'using a motor vehicle in a dangerous condition'.
See link in related information for advice on driving in winter conditions.