the one used by music companies and shops - which will mean acoustic music and singer songwriting

If folk means 'acoustic music' then the Brodsky Quartet are folkies. The whole of Basket of light is folk (including the band compositions) but hardly any of Parcel of rogues (all trad). And, of course, Seth Lakeman definitely isn't folk.

If folk means 'singer songwriting', then Dylan's a folkie; Anne Briggs wasn't a folkie to begin with but became one later on; and Bert Lloyd wasn't a folkie at all (or was he?). On the other hand, Billy Bragg's certainly a folkie, and so's Eric Clapton. Joe Strummer, Kurt Cobain and Jimi Hendrix were all folkies on this definition.

If 'folk' means 'whatever the record companies and the shops are calling folk at the moment', that's not a definition so much as a bag - and it's a bag that could get larger or smaller at any time. In a year or two the wyrd/twisted/hyphen-folk caravan is going to leave town, and the definition is going to contract again by default. Maybe fashion will swing the other way and we'll see the folk racks dedicated to traditional music played on acoustic instruments. If that happened, what would you say that the definition should include?