I'd like to vote for (or possibly against):

'The Streets of London'
'The Streets of Athenry'
'Imagine'

The latter isn't, of course, a folk song - but then neither are the other two!

Oh yes, and let's not forget 'The Constant Lovers' -

"Ohhh, ohhh, ohhh my love is gone ..."

"Yes and you can piss off as well!!!"