Thanks, I was wondering if there was an over lap between the Romans and the first missionaries. I guess not.
Christians arrived with the Romans, but before the Catholic Church in Rome took hold across the continent, the Romans quit the British Isles and Christianity developed independently for nearly two hundred years in England, Scotland, Wales and Ireland; becoming strongest in Ireland.
In the late fourth and early 5th centuries, Irish missionaries established monasteries and Churches in the British Isles (Britainia and Scotland, and Wales), while the "pagan" Angles (later "English") and Saxons were invading and becoming ascendant there. It was not until two hundred years later that the Church of Rome became very active in Britain and the now settled Angles and Saxons received a "full court press", for conversions, from the Vatican.
Some religious novelist ought to write a novel in which the independent Christian churches in the British Isles become the dominant faith there before any great attempt from the Vatican is mounted and after which such attempts from the Vatican do not succeed; leaving a fully independent Christian Church not centered in Rome or Constantinople, from the 5th Century on and long before the later reformation.
The Anglo-Saxon and Irish invaders were pagans.