The "Green Man" entry in the book England in Particular (which I've got on my knee as I type) treats the Green Man, Jack in the Green and the Green Knight as different things, doesn't refer to Lady Raglan and does point out that "green men" can be found in a lot of different countries. It's got definite "pagan survival" leanings, but it's actually pretty cautious. Closing quote:

"THe tree in its deciduous forms symbolises the cycle of death and rebirth, re-enacted each year as leaves fall and grow again. The Green Man has emerged in our time as a symbol of reconnection with nature, of regeneration and hope."

(Note the words "in our time".)

Great book. It's not academically rigorous, but it's not fakelore either.