As far as Cate Blanchett goes, initially I'd be against it, but having seen her play Hela in Thor: Ragnarok, and her own interest in playing villains I could see it.
Seeing as Greta Gerwig has worked with her before, I could actually see Saoirse Ronan as the White Witch. I saw one article that suggested she should play Susan, but the problem with that is Saoirse is almost 30, a little too old to be playing a pre-teen.
If we're expanding our search outside of Britain, I'd also be partial to Dakota Fanning, and her sister Elle, as Jadis and her sister.
Otherwise I'm partial to Natascha McElhone ( The Truman Show) as Jadis, and not just because she's British. I always felt she had the kind of eyes that were perfect for an enchantress, the kind that sort of draw you in and beguile you. Kind of like Cate Blanchett's in a way.