“Where a suit about a particular subject matter between a plaintiff and a defendant is already pending in a foreign court which is a natural and appropriate forum for the resolution of the dispute between them, and the defendant in the foreign suit seeks to institute as plaintiff an action in England about the same subject matter to which the person who is plaintiff in the foreign suit is made defendant, then the additional inconvenience and expense which must result from allowing two sets of legal proceedings to be pursued concurrently in two different countries where the same facts will be in issue and the testimony of the same witnesses required, can only be justified if the would-be plaintiff can establish objectively by cogent evidence that there is some personal or judicial advantage that would be available to him only in the English action that is of such importance that it would cause injustice to him to deprive him of it. ”