1888 British map of Japan and Corea
This is a map made in 1888 by J. Bartholomew who was a famous geographer. Liancourt Rocks (Hornet Rocks) were not drawn in this map but Dagelet island was drawn with the Japanese name Matsu-sima in parentheses. Most importantly, there is a borderline between Japan and Korea (Corea). The same line divided west part of Japan in three districts - Kyushu, Shikoku and Honshu. And the borderline indicated that Dagelet island was Japan's territory, although this is not true but it shows UK in the 1880's thought so. Liancourt Rocks, which was discovered by UK in 1855 as Hornet Rocks, were not included here but if they were, they should be definitely drawn as Japan's territory.