Arguments to take the certificate as conclusive evidence cited by Lord Sumner:
(Best evidence) the court has to inform itself from the best material available
(One voice doctrine) courts and executive should follow the same approach in matters of foreign affairs.
[Other argument suggested (not cited by the court) is when an element of recognition by the UK government is necessary]
The government of Kelantan had not submitted to the jurisdiction of the court for the purpose of the proceedings to enforce the award, either by assenting to the arbitration clause or by applying to the court to set aside the award.
Oppenheim ‘it is the practice of English courts to accept as conclusive statements by Secretary of Foreign affairs and Commonwealth Affairs relating to certain categories of questions of fact in the field or international affairs. In such cases the statement is conclusive, even in face of contrary evidence’
The ‘practice’ has been incorporated in statutes; State immunity act 1978, 21