I am not sure I buy into this. The activities of the mediaeval church have left the records of English folk music and song limited, so the richness of the English heritage is indeed limited in comparison with Scottish and Irish. What is more it is different in type. You can hear the difference between English on the one hand and Scottish and Irish on the other. I daresay if your ear is attuned and you care that you can hear the difference between Irish and Scottish too.

It is admirable if there are new interpretations of English folk music and song (and I don't care what roots they draw on, my interpretations are "folk and roll" which means Afro-American corruption of the English wellspring, but it's what I do) but I am not at all clear that this implies cross-fertilisation between "Celtic" and English.