Almost everything known about early religion in these islands was written down by Christian clergy, and mostly by monks, which shapes what survives and how it reads. Bede is the fullest early narrative and he writes to a purpose: the unity of the church, the vindication of the Roman Easter, and the holiness of particular Northumbrian figures. Irish annals, saints' lives and law tracts preserve a great deal of older material, but preserve it inside a Christian frame that turns gods into kings and heroes and files myth as history.
That leaves the pre-Christian religion of the islands to be reconstructed from objects and from outside witnesses. Classical writers describe the druids of Gaul and Britain, usually while justifying a conquest, and Ronald Hutton's history of the subject traces how little of what is popularly attributed to them can actually be got out of those texts, and how much of the familiar image was invented between the seventeenth and the nineteenth centuries. The archaeology, deposits in rivers and bogs, shrine sites, unusual burials, is real evidence, but it rarely tells you what anyone thought.
The result is a record that is unusually good for the early Christian centuries and unusually poor for everything before them, with a hard edge between the two rather than a gradient. The atlas records that edge rather than smoothing it, and marks reconstruction as reconstruction wherever the evidence will not carry more.