This situation was further exacerbated by Anglo-Saxon invaders who from the mid-fifth century onwards accelerated their territorial expansion, and whose depredations upon the population destabilized the British Church to such an extent that the Church and many of its clergy were driven from eastern and central Britain into the West of Britain, the Highlands of both Scotland and Wales and across the sea to Ireland. Many fled to the region of Gaul we now know as Brittany.
Monasticism reached Britain in the fifth century. Monastic settlements, such as Bardsey Island and Llangadfan, which were established by St. Cadfan in the mid fifth century, were often located in isolated areas of western and northern Britain. However, Ireland, being generally free from the turmoil of the mainland, proved to be a more conducive environment for monastic communities. Around such settlements the ancient Church rallied, consolidated its resources and began to rebuild itself. Indeed, it is likely that St Patrick’s mission in Ireland started from such communities. In 597AD, Augustine began his mission of persuading the much reduced British Church to accept the authority of Rome, and converting the Anglo-Saxons to Roman Christianity. At first the British Church resisted Augustine’s overtures, but eventually, at the Synod of Whitby 664 AD, many of the Bishops were persuaded to accept Roman authority, thus the ancient British and new Anglo-Saxon jurisdictions were fused into one. Over the course of time most of the ancient Church was slowly absorbed into the Church of Rome.
During the Reformation, and after the accession of Elizabeth I, Roman Catholicism, including whatever remained of the ancient British Church, was proscribed and persecuted by law, which effectively brought about the extinction of the ancient Church. And so it remained until the mid-nineteenth century, when a Celtic renaissance took place and a considerable number of committed Christians were inspired to pursue the ideal of the simple and pure spirituality that they perceived in the primitive British Church.