I think it's very possible that the "Whea" could stand for "wheat." The original meaning could be "valley of wheat." Removing the "t" at the end would've made it easier to pronounce since both T and D have similar sounds. Eventually, the dale was shorted to dle. That's just my guess.
Although, isn't "whi" the Anglo-Saxon word for "white"? So it could also mean "white valley."