Matt: "there are a couple of big-name UK folk albums I've listened to recently where it did sound as if auto-tune was being used. ... [they] were very much at the pop crossover end of the folk scale"

This doesn't exactly surprise me too much I must confess. As you say, it seems to have become a standard process for commercial music. Though not being much of a listener to modern folk, I can't say as I've really noticed it - though I could hazard a guess or two at names.