Paul, there isn't a single market. There are different markets with different expectations. The Coppers weren't singing to break into a particular market, they sang what they sang and how they sang because they enjoyed it, and the market they were performing to (even if it was just the family) enjoyed it too. A wider market which appreciated that particular style found them, not the other way around.

If we take another example, Walter Pardon was a singer who adapted his repertoire, and to a degree his singing style, to the demands of his audience in his local pub, who wanted more of the music hall and popular songs rather than the traditional songs which he loved. So he gave them what they wanted, and sang the old songs to himself until he was discovered by the folk scene.

Within the "folk scene" a particular style developed which differed not only from the traditional styles (eg Coppers, Fred Jordan, etc)but also the previous piano-arrangement styles. It was partly influenced by American folk styles, but developed a particularly British flavour. Anyone aspiring to perform in folk clubs learned to perform in that particular style - for many, that is what they mean by "folk".

For those aspiring to more commercial success in the context of the broader music industry, a different style again is required. It's not one which will necessary go down well in a folk club.