existentialist
Tired and unemotional
It's potentially several offences.Yes, for example if you don't clear your roof and or bonnet you can be fined and get points on your licence, I forget exactly what the offence is but it is an offence.
A car with bits that can drop off (ie snow) will represent an offence under the Construction and Use regulations; a car whose lights are not fully visible will be an offence under the Vehicle Lighting regulations, and a car whose numberplate(s) or tax disc (not relevant any more) are not visible will be an offence under the Vehicle Excise and Registration regulations.
ETA: oh, and as Bahnhof Strasse's post points out, a really extreme case could be careless (or, I imagine, reckless) driving, too.