This is a broad category for monarchs who ruled part or all of the island of Great Britain, which is politically dominated by the United Kingdom of Great Britain and Northern Ireland at present.
Great Britain's Iron Age inhabitants were Celtic speakers known as the Britons. The Romans conquered most of the island (up to Hadrian's Wall, in northern England) establishing the province of Britannia. In the course of the 500 years after the Roman Empire fell, the Britons of the south and east of the island were assimilated or displaced by invading Germanic tribes (Angles, Saxons, and Jutes, often referred to collectively as Anglo-Saxons). At about the same time, Gaelic tribes from Ireland invaded the north-west, absorbing both the Picts and Britons of northern Britain, eventually forming the Kingdom of Scotland in the 9th century. The south-east of Scotland was colonized by the Angles and formed, until 1018, a part of the Kingdom of Northumbria. Ultimately, the population of south-east Britain came to be referred to, after the Angles, as the English people, and the Kingdom of England was formed in the 10th Century. The course of history saw England annexing Wales and Ireland in the 15th Century. The 17th Century saw a union of the crowns of England and Scotland in James VI of Scotland, who ascended to the English throne in 1603.
As Harry Turtledove has written few stories involving the British Isles before the creation of England, Scotland, Ireland, or Wales, this category mainly contains monarchs who ruled the various tribes or kingdoms that existed before the creation of those modern countries. Monarchs of the kingdoms of England, Scotland, and the United Kingdom should go in their own own respective subcategories.
This category has the following 3 subcategories, out of 3 total.