Richard, I feel if there are any 'unacknowledged' influences it might be because they either don't exist or they remain unknown. Having said that, The Beatles, IMO, were simply a compilation of many influences and like many bands of their era, the influences go back to the very beginning roots of music. In other words, this would include
every genre of music that came before them. They were influenced by jazz artists like the Chris Barber Jazz band. In the late '50's the then Quarrymen even played a song written around 1917 called "Putting on the Style". While in Hamburg they often covered such songs like 'Moonglow' . A swing number that had often been played by Benny Goodman and Artie Shaw. The list goes on. It was all a natural progression from there I suppose.
Just because they were from England it doesn't mean their root influence came from medieval compositions or Celtic folk music. It was music which evolved from a myriad of influences.
For the larger question, possibly a study of the evolution of the European bard would give you a better answer
