The Roman Church first came to Britain when King Lucius invited missionaries to come to Londinium and teach around 150 AD. The priests Faganus and Dyvanus established a church in Londinium, which still stands, and began their teaching. They first confined their preaching primarily in the cities, and many parts of rural Logres have churches with semi-educated priests who allow semi-pagan practices to continue.
Almost 200 years ago, in 306 AD, the greatest Roman Emperor departed from Britain and conquered the corrupt Roman Empire. He was Constantine of Britain, the only Roman Emperor ever titled the Great, who later was made into a saint. He converted his empire to Christianity and gave over much of its administration to the Church. Since that time, their bishops have held political power (especially in the cities) even though the empire has fallen.
The Roman Church holds its own property. They have a separate judicial system called Canon Law, which oversees legal issues within its membership. They hold property that is often in Free Alms, which means they are not responsible to any secular lord for it. They also hold property by the normal Grant of Fief, like lords and knights, and those who do must supply troops for those holdings.
All the bishops in Britain are under the leadership of Archbishop Dubricus. His own see is outside Logres, in Carlion, but he is as British as a man can be. He oversees ten bishops, each in his own city, who in their turn oversee the priests within their regions. He often travels throughout the realm to check on his flock.