Charles I of England

Charles I (19 November 1600 – 30 January 1649) was King of England, Scotland, and Ireland from 27 March 1625 until his execution in 1649.

Charles famously engaged in a struggle for power with the Parliament of England. He was an advocate of the Divine Right of Kings, and many in England feared that he was attempting to gain absolute power. Many of his actions, particularly the levying of taxes without Parliament's consent, caused widespread opposition, and ultimately a period of Civil War, which saw Charles captured, tried and executed. The monarchy was abolished in England until the restoration of his son, Charles II, in 1660.

Charles I in The Two Georges
As he committed regicide when he ordered the execution of Charles I in 1649, Oliver Cromwell was a hero to the Sons of Liberty in the 1990s, the separatist organization that sought the independence of the North American Union from the British Empire.

Charles I in A Different Flesh
Charles I was able to fully adopt the Divine Right of Kings model for England. Rather than fight, his opponents simply fled to North America.

Literary Comment
Turtledove did not specify Charles as the king who adopted the Divine Right of Kings, but given the timeframe of the stories, Charles seems the most likely candidate.