Britain was founded 586 years ago on the day Merlin was born and was ruled by the First Generation Arthur along with Merlin.
Britain is a another kingdom other than the Bairong Empire that can stand against the Witches without the help of the Witch Hunters. Their last ruler, the 9th Generation Arthur, King Arthur, disappeared; he left behind his wife-turned-Witch who fled with Lancelot du Lac, Knight of the Lake, becoming her supporter, and her daughter, who is now Tasha Godspell's supporter, Halloween.
Britain is also protected by thousands of armies and the twelve famous knights know as the Knights of the Round Table, which is now falling apart since Arthur's disappearance and Lancelot's departure. It is also the country that shows the most animosity towards witches and kills them without exception regardless of who they are. One example was how Guinevere was sentence to execution merely because of the fact she was discovered to be a witch, an unfair sentence that both Arthur and Lancelot show great displeasure about, as well as how the Knights of the Round had pressured Mordred into fighting against West despite how West is one of the witches, along with East, that does not wish to wage war against humans. Overall, this showed the clear difference between Britain and the Witch Hunters, as witches can be accepted as Witch Hunters as well as how some of the Witch Hunters utalize a humane procedure in capturing a witch alive where they are later imprisoned; this is likely one of the reason why Britain, especially among the Knights of the Round, tend to show discontent when asking for the Witch Hunters' assistance since the WH is not too closely tied to religion as Britain. While Merlin states how witches and humans had once lived closely in harmony, it is currently unknown how or why Britain had adopted a religion that had ignited national hatred towards the witches considering since Merlin had been around the politics of Britain since Britain's first king.
It is also the home of the famous Archmage Merlin, the only man on Earth who can use magic and able to manipulate other witches' power as long as they are within his magic circle which is the city of Camelot. however to achive such power Merlin has reconstructed his body into a female's body in order to be able to store mana in his body without poisoning him. He disappeared right about the same time as Arthur did, but returned upon sensing Halloween's presence.