The head of a State's diplomatic mission in the United Kingdom, or the person for the time being performing his functions, shall be deemed to have authority to given on behalf of the State any such consent as is mentioned in subsection (3) above and, for the purposes of subsection (4) above, his certificate to the effect that any property is not in use or intended for use by or on behalf of the State for commercial purposes shall be accepted as sufficient evidence of that fact unless the contrary is proved.