Dereines Isles de la Grant Bretaigne, Isles de Jedares, Isles d’Orient
… with the beautiful heath.
Considering “Dereines Isles de la Grant Bretaigne,” this should be islands off the coast of Brittany. There we find the Île Aval, with a Briton Arthurian legend.
The Île Tristan (Tristan Island), got its name in the fourteenth century, after the Breton legend of Tristan and Iseult (Isolde).