Rosie,
Best overall introduction to English song by far - A.L. Lloyd's 'Folk Song in England'.
Best basic collection of English folksongs - Lloyd's and Vaughan Williams' 'Penguin Book of English Folksong'.
Best introduction to folk ballads - Evelyn K Wells' 'The Ballad Tree'.
All of the above can (and have been) challenged academically, but they are all extremely readable and provide an excellent starting point for anybody wishing to learn about 'folk song proper' - enjoy.
If anybody yawns at your question - feel free to kick them in the teeth.
Good luck- hope you have as pleasant a journey as I have had,
Jim Carroll