Crusader Kings was based on the old RPG game 'Pendragon' so its possible. I have an old copy somewhere, where Arthur rules from Hampshire and it has a map of Britain and the rulers for each area....
...ah here we are
Anglia: Duke Hervis of Anglia (Saxon)
Bedegraine (Around Nottingham): Earl Sanam - vassal of Pendragon
Brun (between Warwick and the Fens): Baron Garmon of Brun -vassal of Pen.
Clarence (betw. Bristol and Gloucester): Duke Galegantis -vassal of P.
Dorset: Praetor (Duke) Jonathel (Roman) -vassal of P.
Essex: Earl Bleoberis de Ganis (Occitan -over saxon subjects) -vassal of P.
Hampshire: King Arthur Pendragon
Hertford (Bedford): Earl Gilbert of Hertford -vassal of P.
Huntingdon (around Bedford): Earl Dafydd of Huntington (Cymric -welsh/briton) -vassal of P.
Jagent (between Devon and Dorset): Earl Tegfan of Jagent -vassal of P.
Kent: Earl Kynniarc (Cymric over Saxon) -vassal of P.
Lambor (Manchesher/Leicester): Baron Blanmore de Ganis (Occitan) -vassal of P.
Lindsey (Lincoln): Duke Derfel of Lindsey
Lonazep (near the Fens): Earl Macsen of Lonazep -vassal of P.
Rydychan (Oxford): Earl Boso -vassal of P.
Salisbury: Earl Robert -vassal of P.
Silchester (Between Salisbury and London): Duke Ulfius (Saxon) -vassal of P.
Somerset: King Cadwy -vassal of P.
Southports (Hampshire coast): Admiral Theoderic (Saxon) -vassal of P.
Sussex: Earl Celyn of Sussex -vassal of P.
Tribruit (North of Oxford): Baron Meilyr of Tribruit -vassal of P.
Wuerensis (Warwick): Earl Artgualchar -vassal of P.
Cameliard (Betw Manchester and Chester): King Leodegrance (father of Guenever) -vassal of P.
Cheshire (Chester): Randle, Dux of the City of Legions -vassal of King of Norgales
Escavalon (Gwent): King Alain -vassal of P.
Estregales (Dyfed): King Lak (Irish) -vassal of P.
Galvoie (Shrewsbury..ish): The Damosels Yguerne, Morchades and Clarissant
Gloucester: Duke Morvid of Gloucester -vassal of King Alain of Escavalon
Gomeret (Perwydd..thingy): King Maelgwyn
Gwaelod (Gwenedd): Gwyddno Garanhir
Norgales (Gwenedd): King Galihodin
Orofoise (Shrewsbury): Earl Gwarthen of Orofoise -vassal of King Belinans of Sugales
Powys: King Belinans of Sugales
Cambenet (N. Carlisle): Duke Escan -vassal of P.
Deira (NE of York): King Wilgils of Deira (Saxon) -vassal of King Barant de Apres
Malahaut (York): King Barant de Apres (better known as The King of 100 Knights [le Roi des Cent Chevaliers]) -vassal of P.
Nohaut (Northumberland): Lady Ethfrida of Nohaut (Saxon) -vassal of King of Malahaut
Roestoc (Lancashire): King Eifion of Roestoc
Benoic (Fife and Angus): Duke Ansirus (Pict) -vassal of P.
Escoce (Marr to Inverness): King Carados (Pict)
Garloth (Berwick): Queen Elaine -vassal of P.
Gorre (Dumfries and Galloway): King Uriens -vassal of P.
Lothian: Stewards of Pendragon (was King Lot) -vassal of P. (Lot wasnt a vassal)
Orkneys (incl Caithness): Queen Margawse (wife of Lot)
Pictish Highlands (Athol, Argyle up to Caithness): many clan lords (picts)
Strangorre (Strathclyde): King Brangore -vassal of King Uriens of Gorre
Devon: Prince Geriant, son of Erbin -vassal of King Mark of Cornwall
Lyonesse (tip of Cornwall): King Meliodas -vassal of King Mark
Totnes (cornwall): King Mark
Cornouailles: Duke Hoel - vassal of King Mark
Domnonie (East of Leon) King Riwal
Leon: King Melaiu
Vannetais (East of Cornouailles): King Conon
Ailech (the two Tir provinces): High King Muirchertach Mac Erca (IRish/pagan)
Connacht: Eogan Bel (Irish/pagan)
Dat Riada (Ulster): Duke Galahaut (Irish christian) -vassal of P.
Leinster: King Anguish (Irsih/Christian) -vassal of P.
Long Isles (Western Isles): Duke Galahaut (Irsih/Christian) -vassal of P.
Meath (Mide): High King Muirchertach Mac Erca (Irish/christian) -vassal of P.
Munster: Crimthann, son of Fedelmid (Irish/pagan)
Oriel (Nth of meath): High King Muirchertach Mac Erca (I/p)
Pomitain (Man): King Marsil (Manx/pagan) -vassal of Duke Galahaut
none of this is probably of any help. But I had nothing better to do today.
Ayeshteni