I am surprised that iSpot gives Great Red Slug as an alternative to Great Black Slug for Arion ater. Have I missed something?
Arion ater Great Black Slug
Arion rufus Great Red Slug
Arion vulgaris ? not sure what this is called
I suspect that Great Red Slug was formerly treated as part of "Arion ater agg.", and the English names are still linked that way - will look into it.
Entomologist and biological recorder
English names may be problematically in the less popular groups.