This is a Probable ID based on what little we can see and items that we know Catherine has.
We think Catherine was wearing her JoJo Maman Bebe Women’s Maternity Princess Line Coat as they left for Anmer Hall.
You can find the coat in a range of sizing at Amazon.com starting at $91.
It is currently available at JoJo Maman Bebe for £69
Visit to the Downton Abbey set