Romano-British is a term to describe the people who inhabited modern day England and Wales, who were under Roman rule. They were conquered by the Romans under Emperor Claudius when in 43 AD he invaded the region (local resistance died out sometime around 50 AD). Due to the general decline of the Roman Empire, in 410 AD the Romans were forced to abandon the province. Quickly authority devolved into chaos, previous to this the only real administrators of the province were the now absent military. Various petty Romanized British kingdoms popped up and began to fight amongst each other, and the Picts began to invade as well.
Culturally speaking, the Romano-British were a Roman faction. They appear when the Western Roman Empire loses their British provinces. The Romano-British primarily use renamed Roman units such as "British Legionaries" and "Coastal Levies" (the latter being the same as the Roman Limitanei), but they can also recruit the legendary Graal knights from Arthurian legend. When they appear, they are an incredibly effective force, and their beginning army often includes very high experience units.
The Romano-British have a mix of Roman-styled infantry and Barbarian units. One of their best units are Graal Knights, a powerful mounted unit, which are very heavily armoured.
The Romano British do not start on the map, instead they emerge if the Roman provinces in Britain are captured. They are unique in the game as the only faction able to survive without owning a settlement, whilst not also being a horde capable faction. Normally, their emergence is triggered by the invasion of the Saxons as the Saxons are required to conquer British regions so as to win the game.