The origins and past history of Captain Britain, is similar to that of his Earth-616 counterpart, however there are a number of facts that have come to light about his origins that are exclusive to Earth-9997.
Through research of the past, a combination of X-51, and Kyle Richmond, a number of facts about Captain Britain's origins had become known. Firstly of those is that the sword which would one day become Excalibur (famed weapon of King Arthur) was forged in the distant future from a star that was once Galactus. This sword came into the possession of Prince Wayfinder, who brought it to Earth's prehistoric past, and somehow through the generations it came into the possession of King Arthur, and later Merlyn the sorcerer.
It was also revealed that when Brian was choosing between the Amulet of Life and the Sword of Death, during the time he was chosen to be Captain Britain, he should have chosen both, a truth that became evident much later in his life.
In present times, either during or prior to the mutation of the entire human race, Captain Britain and Meggan were married to each other, and they continued to fight in Excalibur. After the world's mutation, Excalibur fought against the Grey Gargoyle who turned all Brian's teammates into stone (except Nightcrawler who managed to escape), in a fit of rage, he took the Sword of Death (aka Excalibur the sword) and slew the Grey Gargoyle, not realizing that the Gargoyle himself was the only way he could change his friends back to normal. He spent the convening years getting Grey Gargoyles from other realities to try and free his friends, but none could lift the curse.
Also during this time, Brian was put in charge of raising the son of Black Bolt and Medusa of the Inhumans. He would raised the boy as if he were one of his own, and the boy grew up to become the next Black Knight. Also during this time, the world -- unable to cope with having super-powers due to planet wide mutation -- selected experienced super-heroes to be leaders of their home countries, Captain Britain soon found himself becoming the King of Britain.
Either prior too, or during his reign, Britain's naval fleet became in charge of distributing Russia's wheat reserves to feed the rest of the world. King Britain also came into possession of the Dragon Man android and used it as a steed, and Britain biologically engineered an army of Union Jacks to protect the country. When Crusher Creel was destroyed by the Vision in Washington, D.C. after he killed the Avengers, a part of Creel was given to King Britain as well.
Around the time that the Skull was slowly taking over the United States, Captain America came to Britain to recruit super-beings to help in his fight against the Skull. King Britain committed himself and his troops to the fight and eventually the Skull was killed and his slaved freed.
King Britain returned home, and because Galactus destroyed the Celestial Embryo gestating in Earth's core, the planet's poles shifted and over the next three years the United Kingdom evolved into a tropical paradise. He also opened his home to Medusa and Luna of the Inhumans, and constructed an army of Iron Men based on Tony Stark's Iron Avengers robots.
Later, when Captain Mar-Vell was reborn and on a quest to collect items of power, he came to King Britain to help obtain the Book of Vishanti and the Book of Darkhold, which were being held by Merlin in the Siege Perilous.
Shortly afterwards, his kingdom was attacked by the Tong of Creel, seeking to collect the piece of the Absorbing Man which had been in King Britain's possession for years. In spite of all their resources, they failed in stopping the Tong from getting what they sought.
Sometime afterwards, King Britain proposed to Medusa of the Inhumans to unite the kingdoms of both humanity and the Inhumans. Soon after their wedding, their wedding night came to a dramatic end when Captain U.K. and a Grey Gargoyle from another reality managed to free the members of Excalibur trapped in stone (Captain UK and Gargoyle were really Mephisto in disguise). With Meggan's "resurrection" the validity and where King Britain's true feelings were then in question.
However, later when Mephisto came back with an army of Deviants to take Excalibur (the one thing other than Jude that could kill people in a world where Death had died) in a bid to take over the world, Meggan sacrificed her life disguised as King Britain, giving the real King, Medusa, Black Knight, and Ransak the Reject the time to defeat the Deviants and kill Mephisto.
Following Mephisto's death, King Britain's current activities are presently unknown.
Powers and Abilities
King Britain has the resources Buckingham Palace at his command
King Britain wields the sword Excalibur, which was forged through a star in the distant future, the same star that was once Galactus and is the source of the powerful Enigma Force. Dubbed the sword of death, it can fatally kill anybody. There appears to be no known limitation to who the sword may kill as it has been shown to be able to kill people in a reality where Death has (and thus the concept of) is dead, and are incapable of dying.
King Britain often rides the Dragon Man android like a horse, this android is highly durable, can fly by means of it's wings and can breath fire. See Dragon Man for more information.