I'm five pages late already! Oh well- here's something for the new thread- basically a first attempt at a conlang style development that grew into a map-
In this universe, the first Anglo-Saxon King to convert to Christianity (Ethelbert of Kent) is killed soon after by a rival brother who in OTL died in infancy. With him, the Archbishopric at Canterbury and most Christian-converts are killed, as well as the King's wife- a member of the Frankish Royalty. Subsequent Frankish attempts to conquer England (pulled over four centuries ahead of OTL's schedule) lack William the Conquerer's combination of Nordic and Roman fighting styles, and end mostly in defeat (though by the end of the 8th century Cornwall is a Frankish vassal, owing to support from Irish allies on the other side of England). For the duration of the 7th and 8th centuries, England unites roughly as it does in our world as a non-Christian monarchy. Though there are many attempts to bring the monarchy to Christianity, the Pagan elite slowly solidify their opposition and organization.
From this time, the elites of England (and also Scotland) actively reject the Christian world- organizing a developing their own traditions (for instance, decreeing the use of an alphabet intended to look like Runes rather than the Roman script). Jews and Gnostic Christians are given refuge on the island, and in the following centuries an influx of Pagans from Scandinavia, Lithuania, and even Russia gives the island a bit of a multi-cultural flair, developing it quicker that OTL.
Though William has been butterflied, in 1187 amidst the Crusades a Norman King receives the Pope's blessing in conquering Britain, and does so successfully. Right from the start, the Norman King's must balance their desire to please the Pope with converts and their desire to maintain order over a now more staunchly non-Christian population. By 1400, an informal constitution allowing the local 'Ossians' rights to their customs is in place, and Scotland (an Ossian majority place) is an independent ally. The Irish hate England far worse than in OTL, and Norman King's efforts to take care of revolting Irish and revolting Ossians ends up in the resettlement of quite a lot of Ossians. By the 1400s, enough Ossians have risen through the (somewhat meritocratic) ranks of the aristocracy to make the Pope unhappy.
In 1789, Fresh off the conquest of the new world, the triple monarchy of Francia-Beligica-Germania (Belgica being the Low countries), uses its wealth to have a much bloodier conquest and holy war towards conquering Britain. Most of the existing Christian populations side with the Ossian majority (even uniting under an Ossian leader who acts on the King's behalf), but with enough cash the continental forces are successful (Iria, or Ireland, is quite happy to be freed).
As the triple Empire breaks up at the dawn of the 20th century, various plans are put forward for Britain's future. Though long campaigning for local-rule together with their Ossian counterparts, the Christian minorities eventually buy into politicians calling for an independent Christian homeland on the isles.
Dumnonia,
Wales,
Isle (of Wight),
Cornwall,
Atrebatia, and
Durotricia were eventually chosen to be, along with Iria, part of a Christian homeland. Dwicadia, as it came to be called, separated from the rest of the new Republic of Britain.
Following this event, there was a painful period of rioting, homegrown death squads, and reignited religious division on both sides, resulting in significant population transfers. Dwicadia suspended its own constitution for the first thirty years of rule, and since then has remained a quasi-junta state with a state religion. Though far more linguistically diverse than OTL, the entirety of the British Isles came under one official language (based off of the London dialect), though don't tell anyone that. Despite being the same in spoken and vernacular forms, both sides changed their language tremendously. The 'Royal Language of the Christian Court', or Royeauish embraced Latin and French and was written in the Latin Alphabet, while the 'Language of the land', or 'Angalish' reconstructed ancient words and tried to renew their day-to-day use*, while being written in the Modern Runic Alphabet.
The Irish came close to outnumbering the Dwicadians in East Dwicadia, and more resistant to the idea of a national language, or Royeauish. In order to maintain authority, the government in Christplace held sham elections and began quietly implementing a reign of terror on the Irish countryside. As the Irish independence movement broke out, the Dwicadian government responded in full force and brought genocide, ultimately needing to be put down by the Republic of Britain.
The Republic of Britain today stands as one of the wealthiest and most densely populated countries in Europe. Despite having held no colonial empire, is weight is felt around the world. Though a bit corrupt and prone to communal tension, it is democratic and its (still fairly divided) communities live in peace together. It has pursued a path of non-alignment with other states, but is gradually opening up, both to a continent that has been somewhat hostile, and to a wider world.
*but with a modern pronunciation. Proto-Germanic may have pronounced 'frijaz' with a glide in the middle, but the modern reintroduced word is pronounced like 'fridge'-az.