The Funkle Brothers (Simon and Gar) might disagree. It's actually in Eb major mixolydian (I think), with a brief sojourn unto Gb. Could actually be C minor. Vaughan Williams' chords are all over the place, but the solo line outlines an Eb chord in its first three notes, and the piece ends on an Eb chord. No Dbs in the solo line, very few D naturals in the organ part.
Tom