Babel is now available for preorder worldwide on iTunes except for in the US and Canada; it will be made available on iTunes in those countries in early August.
Just so you know, when you preorder hardcopies of the album (CD or vinyl) through the Mumford & Sons official webstore, they come with a digital download. I’m including that information because I also received this question:
You can find details about the exclusive mumfordandsons.com preorder here, but know that you are ordering hardcopies of Babel that also come with mp3 downloads
Thanks for the questions!