Notable Sorcerers of British Mythology (other than Merlin)
King Bladdud, from Historia Regum Britanniae. Father of King Leir and Grandfather of Gonoril, Regan and Cordelia. The earliest known necromancer in Britain.
[...]
Celidoine, King of Scotland and North Wales, son of Nasciens and ancestor of Sir Lancelot and Galahad. Buried in Camelot.
From the Red Book of Hergest Welsh Triads: Math ap Mathonwy, King of Gwynedd, brother of Don, and uncle of his protege, Gwydion, the magician-trickster hero of the Mabinogi. Amongst other things, punished his wayward nephews for raping Goewin by shapeshifting them, tested Arianrhod's virginity with his wand (which she failed), and is co-creator of Blodeuwedd, the flower-bride of Lleu Llaw Gyffes. Uther Pendragon, King Arthur's father, who mentored Menw, one of Arthur's own enchanter-knights. Infamous for using shapeshifting to seduce Igraine, siring Arthur. This triad implies Uther himself was a practicioner of the magical arts and has his own apprentice, with the assistance of Merlin in Historia being Geoffrey of Monmouth's spin. Gwythelyn the Dwarf. Unknown, but his nephew-protege, Coll ap Collfrewy, is one of the mighty swineherds of Britain and the owner of the magical sow, Henwen.
From Iolo Morgannwg's own dubious triads (so take them with a grain of salt): Idris Gawr of Merionydd, of Cadair Idris fame. A huge giant learned in poetry, astronomy and philosophy, who's throne/chair is a mountain said to be able to grant poetic skill or madness. Gwydion fab Don, the trickster figure of the Mabinogi and student of his uncle Math. The Milky Way Galaxy is said to be his fortress. Gwyn ap Nudd, Lord of the Wild Hunt and King of the Fairies of Glastonbury. King Arthur's cousin and huntsman. Doomed by Arthur to fight Gwythyr ap Greidawl for the hand of Creiddylad until the End of the World.
*(Not included are Klingsor and Gansguoter of the German Arthurian Tradition)
It is very notable that many of these Sorcerers are Kings, lordly rulers in their own right.