It is fairly clear that a popular defintion hasn't been settled. It's apparent that Dylan is a central figure in the folk revival, which is itself a common shorthand for 'folk'. He wouldn't qualify in my favoured definition though I can why he would unquestionably fit other's; I'd view some of his later electric stuff scoring more folk points than the earlier.

'Can mean' would be more useful than 'will mean' admittedly. An individual popular music radio presenter might introduce a band as folk entirely on their appearance (I once heard Dexy's Midnight Runners introduced as such) while Radio 3 tend to blur the boundaries including Bartok, RVW, Gorecki as classical/folk hybrids, at least by comparison, rather than classical romantic or modernists.
Billy Bragg would only be excluded to prove a point as his subject matter, delivery and any discernable nuance apart from the fact he a) isn't dead, and b) came to folk through the punk community, would suggest he's very much English Folk. Experience suggest English Folk enthusiast tend to see him as such.

Problematisation is inbuilt in deciding who is folk partly because it's a strategy of the left ('everything you thought is wrong') and the left have cornered strategies of analysis since the term folk became popular. The other difficulty is it's proponents have decided a framework that limits the range of folk to fit an agenda; top down, middle class values examining proletarian ones, set taxonomies and categorisation, the early/mid C20th enthusiasm for neo-romanticism and arcadia and a privileging of old world music dissemination at the expense of the new (I exclude dance and lore without prejudice).

For these reasons (and more) I feel who is folk is still up for grabs.