This is a gem! The English Folk Dance And Song Society Take 6 Archives has digitized six major collections of folksongs.
From the site:
Welcome to the Take Six website, a searchable database of the manuscript archives of six of the UK’s most prominent folksong collectors. Each of the archives have been completely catalogued and digitized, and you can search the catalogues and view the digitized documents here. Most of the documents are songs and tunes, but you can also access other manuscript items such as dances or correspondence.
I will be spending many hours here!