The modern British folk revival has always had a leftish slant, influenced by American stars such as Pete Seeger and Woody Guthrie. But lately the quasi-fascist British National Party has been trying to move in on the folk scene, and now London’s Conservative mayor, Boris Johnson, has organized a folk extravaganza in Trafalgar Square for St. George’s Day (he’s England’s patron saint). Some top folksingers aren’t happy with this new audience.