there seems to be musically a melodic folk style that defines that which is normally accepted as folk style in the uk.
on analysis this involves the use of the dorian and mixolydian scale as well as the major scale not many other scales are used, one exception being one song of dave goulders that use the locrian scale, melodically they generally appear to stick wthin certain melodic boundaries , unlike jazz they do not generally involve improvisation.
as a general rule most songwriters writing in this uk contemp folkstyle, avoid twelve bar blues, occasionally there are exceptions.often musically they avoid diminshed chords.
to suggest any song is a contemporary folk is as daft as some of the twaddle we get from a regular poster on this forum   " we are all going on a summer holiday is a contemporary folk song", no it is not it is a naff pop song.
as for style of writing there semmes to be less constriction that the melodic boundaries that are often used