In the times of old Sosaria, the land that now is Britannia was only one of the four continents of old Sosaria, named The Lands of Lord British. The other three were The Lands of the Dark Unknown, The Lands of the Feudal Lords, and the Lands of Danger and Despair, later known as the Serpent Isle. When the Stranger defeated Mondain the Wizard in Ultima I, the world was thundered apart, and the other three continents were thought lost. With the one known continent totally altered by this event, the Lands of Lord British were rechristened Sosaria.
By the time of Ultima III, Sosaria was under attack by Exodus. When the Stranger and his companions defeated this new threat, a huge cataclysm re-shaped the land once again, into its current formation. Only shortly thereafter, the Kingdom of Britannia was declared.
From the beginning of the Quest of the Avatar to the time of the Black Gate, the lands changed little, although various islands seem to have a habit of appearing and disappearing due to volcanic or seismic disturbances.
Britannia is a world full of interesting places, cities and areas. Note that "Terrain" covers the various terrian forms found in the land.
As was transcribed in the pages of http://ultima.wikia.com/.