Thomas me ol' Rhymer,
I smiled to myself as you wrote....

'I have to agree with you on the "Stairway to Heaven" nomination... The phrasings and the form are certainly derived from English folk...'

...and I'm sitting here typing with Sheffield's Keith Hinchliffe CD, Carolan's Dream (wonderful!) playing - track twelve is the title track 'Carolan's Dream'.
(Do try and get a listen sometime.)
As you probably know, Turlough O'Carolan was a blind Irish Harper (1670-1738)
That particular Zep tune rings out in the A tune part of 'Carolan's Dream'
In line one 'There's a lady who's sure'....then (particularly) line four 'and she's buying a stairway to heaven' - which even the B part returns to.
Irish music (Celtic if you prefer) I suggest, not English.
By the by, surely no coincidence then, that another 'Harper' (OK, in name...Roy Harper) was a major influence on Zep (I believe! Correct me if I'm wrong)
in that he spent a load of time with them in remote parts of Wales introducing (?) them to such Celtic music.
What do you think?
Cheers.