"people are composing folk songs all the time in the uk,"
In order for a song to become a 'folk song' it has to be got at by the folk process' - even by roud's re-definition it has to be sung by recognised folk singers who have been part of that process
Nobody can compose a folk song any more than they can compoe an Elizabethan Madrigal'

"I doubt many people could memorize "
Sorry Jack - not the case
Non literate Travellers (even blind ones) have been the saviours of some of our longest songs and ballads - John Reilly, Mary Delaney and Martin McDonough being prime examples of singers with phenomenal memories
Mary claimed to have being to meomorise a ballad after three hearings - Non-literate Martin McDonagh sang a six minutes version of Young Hunting for Tom Munnelly without mistake or hesitation - he told Tom he had not sung it for forty years
We tend to underestimate our singers
Jim