Howard Jones and a few others have already pretty much nailed this one, but the Peter Bellamy question posed above made me wonder why PB was considered a 'folk singer' at all:

Because many of the songs he sang had a certain provenance?

Because he sang them in a particular style, and accompanied them with a concertina?

Because he performed on the folk club circuit?

Then you have to consider whether, if PB and (say) John Denver were both 'folk singers', the term has any useful meaning - even when restricting it to the folk revival of the late 20th century. The trouble with adopting musicological criteria for a definition is that we can't even find common ground between the British Isles and the USA (where our 'traditional folk music' is usually described as 'Celtic'), never mind the rest of the world.