I see it's very old chestnut time (again).

This "policy", decided by the members of one club for that venue alone was to encourage performers to draw material firstly from their own background and experience and to sing it in their own voices in a language or dialect that they understood and used.

Martin Carthy (among others) has acknowledged the value and importance of this policy (whether instigated by Ewan MacColl, Peggy Seeger or A N Other) at that time because it encouraged the exploration of indigenous English music rather than tedious copying of the repertoires of the then prominent US artists.