I love, love, adore Hadley. It was one of my top 5 names, but DH poopooed it.
B/c Hadley is such a strong name, I'd go for something much, much softer, and very feminine, but still keeping with the traditional English feel.

Hadley May
Hadley Rose
Hadley Amelia
Hadley Daisy
Hadley Violet

Hmm...seem to be stuck on a flower theme. Probably again b/c DH also poopooed them as well.

Good Luck, I'm sure that whatever you choose will be just right.