The British Isles as a single unit is most useful for one thing: the Irish monastic tradition and its reach. Irish Christianity organised itself around monasteries rather than around bishops in cities, produced a penitential discipline of private confession and assigned penance that eventually spread through the whole western church, and sent its monks abroad in a movement its own literature called exile for Christ. Columba to Iona, Aidan to Lindisfarne and Columbanus to Gaul and Italy are the best documented cases.
That expansion met the Roman mission working north from Canterbury, and the point of collision was the date of Easter. The two traditions used different tables for calculating it, so in a mixed royal household the king could be celebrating the resurrection while the queen was still fasting in Lent. Bede treats the dispute at length and presents its settlement at Whitby in 664 as a decision for universal practice over local custom. The Irish churches conformed over the following century, the community on Iona among the last.
The smaller islands are the reason the atlas keeps this entry at all. Man had a Norse and then a distinctively Manx church history; the Hebrides produced a Gaelic devotional tradition of prayers and charms for ordinary work that was collected in the nineteenth century; and Ireland's religious history is its own rather than Britain's. Material belonging to the whole group needs somewhere to sit that is not Britain.