I don't think there's a legal requirement to provide specialist security staff in supermarkets, even at night, although the manager should receive security training as part of his general training. In reality, of course, it makes sense for such places to provide security cover, even if that's only a patrol van around the car park every twenty minutes.
There is nothing to stop your daughter and her colleagues joining a trade union if they wish. There's more about that here:
http://www.worksmart.org.uk/unionfinder/compan ies.php?iny=55