Catalysis of the reaction: 3,3',4',5,7,8-hexahydroxyflavone + S-adenosyl-L-methionine(1+) = 3,3',4',5,7-pentahydroxy-8-methoxyflavone + S-adenosyl-L-homocysteine + H+.
8-hydroxyquercetin 8-O-methyltransferase activity