Having looked at all those above, and more, I concur with jojofolkagogo - 
"www.ukfolkfestivals.co.uk"
gives the most festivals and is easiest to follow.
http://www.ukfolkfestivals.co.uk/england.php#May 
the "Big List" gives everything but you can filter that by
\ Scotland \ England \ Ireland \ Wales

Living Tradition
gives nearly as many but I found harder to read
- the whole UK, by date
http://www.livingtradition.co.uk/festivals/festival-listing  

This year fRoots is linking to the Spiral Earth site which hasn't been updated from 2017.
http://www.frootsmag.com/content/festivals/ 


The Festival Calendar
lists about 25%
https://www.thefestivalcalendar.co.uk/country-folk-festivals.ph 


efdss
has about 50% but might be better focused for some?
https://www.efdss.org/whats-on/folk-festivals-2018 

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