Since two of his sons are Percivale and Lamerake of Wales (or, de Galis), his Isles could be in or near Wales. Anglesey and Holyhead Islands off the coast of northern Wales would answer this description.
Study of the works of Chrétien de Troyes gives an alternative interpretation. In notes to line 419 of Perceval, both D.D.R. Owen and Ruth Cline identify the Isles of the Sea (Owen’s translation), also known as the islands of the sea (Cline’s), as the Hebrides, Cline citing the authority of hilka. In the passage including line 419, Percivale’s mother tells him that his father’s name (which Chrétien does not reveal) was known throughout the Isles of the Sea; again, in and around line 4093, Gawaine remarks that he has never known or heard of any knight from all the islands of the sea whose feats could rank with Percivale’s.
Apparently the same islands are meant in both passages. The context of these references, coupled with the surprising number of characters Chrétien includes, especially in Perceval, surnamed “of the Isles,” causes one to wonder whether the islands in question could conceivably be the British Isles themselves.