It would be nice if the symbol of the insular Christian faith was a Celtic cross.
Speaking of which... any Druidic holdouts in ex-Roman Britain? I'm sure Ireland would still be thoroughly Druidic, as 495 is about the time St. Patrick went there.
Religion is one area that is hard to figure out exactly. When the Romans left in c410, many of the Romanized Celts who had been Christians abandoned Christianity as one aspect of "Romanness". Many of the aristocrats remained Christian but turned to Pelagianism. Cornwall, the lower Midlands, and South Wales seems to have been a bastion of Roman Christianity and Roman culture. For now, I'm coupling 'insular' religion with 'Briton' or 'Irish' culture. I figure that North Wales and a lot of North Britain practiced druidism, and there were also some Near Eastern cults located around the more Romanized areas like Hadrian's Wall, London, and other places that had garrisons for a long time. Shrines to Mithras were discovered in these places, and there is a suggestion that in Cornwall, Tintagel was the site of a cult to Hercules. In 410, a lot of this would still be going on, and even by the 490s I imagine that there was a pretty wide spread of beliefs in the supernatural. Then we have the Saxons, Angles, and Jutes who bring in something close to Norse Paganism. I'd like to leave it up to personal choice for the player in particular, but there is not a strong organized religious body until much later, when you can have a event like Augustine of Canterbury showing up to get Britain in line with Rome.
So there is a lot of room for complex religious stuff in this mod. Right now though as I am creating province files I'm leaving most provinces as "insular". But this will change later. For the event mod that will be an add-on to Matter of Britain, there will be a lot of religious and magical stuff going on.