After trying, and failing, to subjugate the people of the British Isles, the Romans left. Grateful to the island's mutant population for their assistance, mutants, or magi as the common folk knew them then, were given positions of prominence. Needing to fill the power vacuum left by the departing Romans, the mutants raised the first Arthur to power to unite the isles. The British Isles became a haven for mutantkind. The inhabitants rarely stepped outside of their shores in force until the Inquisition began to burn mutants at the stake as witches.
Under Arthur IV, the Kingdom launched a crusade of their own, landing in Northern France and pushing south. After conquering France, the army turned its eyes on Rome, but the Church sued for peace, turning over the leaders of the Inquisition to Arthur in exchange for his promise to leave them alone. What happened to those men is unknown, but while isolated inquisitors continued their gruesome work, it was the end of the organized Inquisition.
The Kingdom, now composed of the British Isles and France, continued to play haven for mutants from all over the world. Home to some of the greatest minds in the world, the growing country became a center for art and science. Their many mutants with useful powers turned their fleets into a trading powerhouse. The country gradually became a constitutional monarchy as the kings and queens gave over powers to a Parliament.
In the present day, and with the concentration of mutants the Kingdom has, it's hard to find someone who isn't at least related to a mutant and their X-Gene carrier percentage is much higher than the global average. The current monarch, Arthur XI, is generally beloved, though he's starting to get old and there is much debate as to the need for a monarch holding the throne. Mutants still hold positions of power without force of mysticism and many of the noble families quite proudly trace both their lines and their powers back for generations. The Kingdom is still one of the most mutant friendly countries in the world and while a few other countries have adopted some of her policies in hopes of bolstering their own mutant populations (for the obvious power and advantage they provide), mutants in many countries continue to be victims of prejudice. Several "pro-human" groups try to operate in the Kingdom and there is some concern that they're gaining traction among some of the poor.
Something strongly mystical still hangs over the islands though and it's often thought that the blessings of Avalon remain with the citizens of the Kingdom. It is also the only country in the world to have a formal embassy with Asgard (though the Asgardian ambassador comes and goes quite frequently). At least one dimensional incursion has been recorded in modern times as Limbo attempted to force itself into York. It was repelled, but an agency, Lighthouse, was put together to monitor for additional incursions under the watchful eye of Captain Britain. York is still recovering and there are a healthy number of refugees elsewhere in the island.
There's more that could be explored like what this meant for the British "Empire" and such, but my brain focused on the central locations. Safe to assume that, other than France, most modern countries still exist. Year is "now-ish" for the purposes of technology, etc, but with the appropriate level of Marvel super science in the mix.
