New York, April 17—AP. A plot threatening the existence of the Episcopalian hierarchy of Great Britain was believed, on good authority, to have been uncovered here today. The alleged instigator of this nefarious scheme, whose motive cannot as yet be determined, is suspected of having urged a prominent photographer of this city to falsify, in some unrevealed manner, a certain document whose exact nature (in the interests of domestic security) remains a carefully guarded government secret, kn...