Catuvellauni
The Catuvellauni were a Celtic tribe which existed in Britain before and during the occupation of the country by the Roman Empire in 43. (AUDIO: The Wrath of the Iceni) During the invasion, they were led by King Caratacus. (PROSE: Byzantium!)
When Boudica, the Queen of the Iceni, encountered the Fourth Doctor and Leela in Norfolk during her revolt against the occupying Roman forces in 60, she inquired if they belonged to the Catuvellauni. Leela told Boudicca that she was of the Sevateem, who the Doctor claimed were located "as far north as you can go and then some." (AUDIO: The Wrath of the Iceni)
The alternate dimension of Avalon was made for the Catuvellauni by Gallifreyan Interventionists to escape Roman invaders. This then caused the society of the Catuvelani to become closely based on that of Gallifrey, such as having a High Council that meets in a hexagonal room and a Castellan. (PROSE: The Shadows of Avalon)