Im pretty sure it is perfectly legal, although its use might be controlled due to rules designed to prevent pollution of watercourses etc. certainly it is the main constituent of bordeaux mixtures, much loved by organic farmers as its one of the few things they are allowed to use.
Animals could be poisoned but I think the use on land is 1;100 (dont quote me) so it would likely need to be a cumulative exposure. Using on resting land might be the best option(aside from improving drainage which would be a more permanent solution albeit an expensive one!)
Some breeds of sheep are more prone to copper poisoning than others eg texels are and some other euro breeds and north ronaldsay especially
Not sure where you are in UK, up here I would be checking out Scottish agricultural college and vets re best approach to take and SEPA re watercourses