Forestan, Foreston, Horeston, Sorestan, Soreston
According to the Vulgate, Sorestan borders Norgales (North Wales) “par evers” Sorelois. It’s uncertain whether this means Sorestan was between Surluse and Norgales, or Surluse between Sorestan and Norgales, or both Surluse and Sorestan bordering Norgales; the last is more likely, and Sorestan be approximately the rest of Chester excluding the peninsula proposed for Surluse.
In Vulgate V, Morgan, Sebile, and the Queen of Sorestan kidnap Lancelot. This seems to be the episode Malory gives in Boox VI, although Malory has four queens: Morgan and the Queen of Norgales, Queen of Eastland, and the Queen of the Out Isles. They kidnapped Lancelot and brought him to Cart Castle, and tried to make him choose one of them as his lover, but Lancelot escaped. It seems likely that the Queen of Sorestan should be identified with the Queen of Eastland, and, thus, that Malory’s Eastland is the Sorestan of the Vulgate.
Malory’s Eastland, on the other hand, might for convenience’s sake be considered separately from Sorestan. Using this theory, Lincolnshire might be called Eastland. Or the name might be given to the easternmost bulge of the island, including Norfolk, Suffolk, and Essex (which name, of course, derives from “East Sex”). Eastland might, indeed, include all the counties listed as raised by Mordred in Malory XXI: Kent, Southsex, Surrey, Estsex, Southfolk, and Northfolk, which would all be sub-subkingdoms.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470