They are called Digestive Biscuits only because they are easy to digest...Oh, I've had them...Carr's are the ones we usually get. They do have wheat germ or whole wheat in them, though, hence my joking...
We call them "diggy" biscuits, though, after hearing them referred to that way in an old British movie..."Strangers on a Train", I think.
I was actually eating some digestive biscuits (McVities) and chatting the the old Hole when got the idea that it was what Tolkien was probably talking about...
I even said something about it way back then...OT probably remembers...
I'll bet you anything he was eating them and writing his book and probably enjoyed them (they are popular!)