As stated, what we know about British Paganism doesn't amount to much. This page will be used primarily at the players' leisure to flesh things out a bit on their own, making their own religion for themselves if they so choose.
That said, there are some boundaries as lain out by Greg Stafford in the main book. They involves a progression of what the religion is and how it looks. He uses some real world knowledge and a lot of writer fiat to come up with it.
The Fae of Pendragon are much like the Tuatha de Danaan of Irish legend. The term is avoided due to the very Irishness of the term but the similarities are many. The Fae were the first living beings in Britain, according to the Pagans, and they worked with The Dragon to create the islands and everything in them. Their leader was Alba, a sort of mother all-goddess. The Dragon itself was less an actual dragon and more a manifestation of cosmic force, a collective manifestation of forgotten things older than even the Fae.
Heathenism is the sort of general nebulous earth worship mentioned in the main religion entry. It was brought over by the Picts, who are still primarily Heathens. The Picts were born on Albion but were never quite of Albion. Their greatest deity at the time, whose name is forgotten or never spoken, slew The Dragon. When The Dragon was slain, The World was separated into the one we know now and The Other Side. The Fae were horrified by these events and most moved on to The Other Side. Those that stayed were less powerful than those who passed on. Heathenism evolved as an amoral, impersonal religion. Morals begin and end at survival in the face of the hardships inflicted by nature.
The Britons, or Cymri, arrived (or so the legend goes) with Brutus, a warrior of the Trojan War and follower of Aeneas. Hence the name of Britain: Brutus-Land or, now, Britain. With Brutus came the ancient civilization of Troy, with its attendent priests and laws and ritual. As they drove the Picts far into the northern reaches of the Isles, they co-opted many of their gods and beliefs. This is the British Paganism of the game's timeline.
Gradually, this imported priesthood became the druidic organization of well-known legend. The druids harnessed the spirits of the Picts, morphing them from amoral, impersonal forces into anthromorphized deities operating within a moral framework with right and wrong, good and evil. The druids themselves were primarily concerned with striking a balance between this world and The Other Side. Druidism was outlawed under Roman rule but, when the Romans finally fled in 400AD, it was revived.
The hierarchy of British Pagan "officials" is extremely decentralized and barely qualifies as a hierarchy at all. The relations between members of different pagan traditions depend upon mutual recognition and respect. The druidic organization is the baseline for comparison when it comes to British Paganism.
The pagan deities do not fall into neat lists or categories. Different parts of Britain prefer their own deities, deities which may be identicial to one another except for name and minor details. One village's Lugh was another village's Nudd. They also tended to get folded into amorphous nature spirits, muddying things further.
God of fire, sun and heaven. Ruler of the Land of Youth.
Lord of the Underworld, riches, ancestor of animals.
Keeper of the secrets of the night, lord of wisdom. His assistant is Gwydion, Druid of the Gods
Don, AKA modrn, Anna, Briigid, Arianhrod, Rhiannon, Epona, GaiaEdit
The First Goddess, worshipped as many aspects. mother, Fire, Moon, Horses, Sovereignty, Earth. Wife of many, mother of the rest.