A better guide might be, would you adopt an accent if you were singing in that region? Americans frequently have a stab at what they think is an English accent (usually cockney) and most are toe-curling Dick Van Dyke affairs. Basically, why bother? If the material is strong enough it'll carry your own voice and if it's heavily inflected, should you be attempting to sing it anywhere if you wouldn't on its home terrain?

As for the universal folk revival voice, it undoubtedly exists but I suspect singers are more aware of avoiding its perils than they once were. Peter Bellamy took the idiom to such a mannerist extreme that it became almost a trademark. Anyone who follows will have to negotiate PB's interpretation.