The British tradition, with a tiny handful of exceptions, is an unnaccomanied one, because, I believe, it is a strictly narrative one.
There is no reason whatever why accompaniment shouldn't be used for folk songs - as long as that is what it does - accompany.
Too often the instrument is either too loud, so the words are lost to the listener, or too intrusive and distracts the attention.
There is an argument (Netl, I think) that a combination of song and crude instrumentation (even the sound of the work itself) accompanied some work songs; believable if you listen to Lomax's recording of convicts chopping wood.
Jim Carroll