"The Eagle of the Ninth," by Rosemary Sutcliff, is the first in the trilogy, its characters the ancestors of those in its sequel, "The Silver Branch." The final volume, "The Lantern Bearers," takes place during Vortigern's rule, in the late 5th century, in approximately the same area, between Aquae Sulis (Bath) and Rutupiae. Almost my favorite historical fiction trilogy. Rated for young adults.
The Silver Branch, the first in the Roman Britain historical fiction trilogy, by Rosemary Sutcliff, is based on the particularly precarious position of Roman emperors stationed in Britain. The two lead characters (cousins), demonstrate tremendous loyalty and heroism, although not as they had originally intended -- in the regular army.
Marcus Flavius Aquila was raised in Rome while his father and uncle fought in Britain. The Uncle settled down there after his years of soldiering, but Marcus' father, who fought with the Ninth Hispana, disappeared. In "Eagle of the Ninth," by Rosemary Sutcliff, Marcus, dissatisfied with his life in Rome, wants to follow after his father and uncle, and gets his wish as an auxiliary cohort.
Romans abandon Britain at the beginning of "The Lantern Bearers," by Rosemary Sutcliff, because they know they are doomed, leaving Aquila, the deserter, behind. He lights the lantern at Rutupiae the night the fleet departs thereby starting a legend.