Therefore, we get what ever kind of reports about people thrown by snowballs, and even snowballs (a large one) was left on the centre of the road blocking the way etc.
and lastly, we have The etiquette of snowball fights.
Well, we all know that snowball fights will occur when snow comes, but we don't know that we may be charged for "Assault" on somebody if we throw one at them, don't we?? Offences Against the Person Act 1861, Section 42 Common Assault and Battery says so. By the time the "evidence" of the assault which is the snowball is brought to the court, if the police or SOCO knew which snowball it is used, it will already melted away.
Links from BBC.