After the period of emmigration from Britain (450s to about 500), most emmigrants landed and settled in Armorica (Little Britain) and the Cantabrian coast of Iberia. From about 465 to 838 these lands are known as the Cornubian Empire. While these territories recognised the suzereinty of the Dumnonian High Kings (who were at this time styled "emperor"), in fact Cantabria and Armorica were autonomous, though Armorica less so. Cantabria was lost to the Moors in the eighth century; Armorica was lost to the French in the sixteenth.
Dumnonia had very little to do with its Cantabrian territories, other than to send soldiers at times of need. Two High Kings were crowned in Ovetum, but otherwise exercised little to no control over local issues. Three legions sent by Gerontios Orgosaxo helpped the Austurians, Visigoths and Basques stem the Moorish flood at Ovetum (Oviedo) in 718. Thereafter, Cantabria passed to Alfonso I of Austurias and passes from Dumnonian history. Now, I suspect the only influence the Romano-British had was some linguistic remnants, as there were already Romance speaking Galicians in the area.
Armorica is a different matter entirely. After the emmigrations of the fifth century, the land became thoroughly Romanised (again) and remained politically attached to Dumnonia and then Kemr until 1532, when the French were kicking the British out of the country. Even after this time, Armorica remains culturally, linguistically and religiously oriented towards Dumnonia.
End of the Empire
The Cornubian Empire comes to an end in 838 when Constantinus v, High King of Dumnonia and last Emperor of the Cornubians, accepts the suzereinty of Gereint IV, Prince of Cambria. This makes both Dumnonia and Armorica part of Kemr: Dumnonia becomes a province, Armorica a subrpovince (in 900). Internally, there is little difference in administration since Constantius v continues as High King, and Armorica's internal structure is left intact.
After 1532, Armorica (Cornouaille) becomes a dukedom of France, a crown territory ruled personally by the King of France in the person of appointed dukes. Since that time, revolts and riots are not uncommon, though independence is not regained.
In ca. AD 425- pagan religious dissenters based in Dariorigum settle islands in the British sea. AD 1220- Arvorec settlers colonise the land between the Rance and the Couesnon rivers.
Chief Cities of the Empire
According to the Map, the cities of Dumnonia are: Isca Dunorum, Dourovaria, Trebellio, Drouronemeton, Pednosancto, Acquesoules, Sorbiodunon & Ouectis Belgorum. The cities of Cornouallia are: Sancto Malo, Sancto Brieucco, Porta Britonum, Dol Britanorum, Pontaballio, Condathes Rodones, Namnetio, Vanetio, Doriorum. The cities of Brigantia are: Brigantio, Lugdunum, Bergidio, Ogusta, Ovetum, Iria and Tudis.