British Democratization
A history of democratization in England, Wales, and Scotland, plus recent history of Northern Ireland.
Note that there is a separate page for Ireland.
Constitutional Monarchy
There is no formal constitutionality to the English monarchy. The current state was achieved through centuries of minor constitutional adjustments. See also the history of how the separate kingdoms of England, Scotland, and Ireland were bound together into a single United Kingdom.
In 1215, English nobility forced John to agree to the terms of the Magna Carta. This proto-constitution limited royal powers. (Note that this is technically just the kingdom of England.)
When the Parliament enthroned William and Mary in 1689, it institutionally assumed the power to depose and raise English kings. (Note that this is technically just a personal union of the kingdoms of England and Ireland.)
Civilian government effectively emerged through the Hanoverian succession. Walpole, considered to be the first British prime minister, held significant influence over George I. This trend is reinforced by Pelman's demonstration of coercive power over George II in the crisis of the short-lived ministry. Finally, Goerge III was largely absent (and/or restrained from power due to insanity), cementing civilian rule in England. (Note that this was all technically just the kingdom of Great Britain.)
Note that civilian here does not imply democratically elected. The Parliament is composed of an elected House of Commons and an ennobled House of Lords. There is only a norm, established in 1963, barring a lord from being prime minister. Furthermore, many 'elected' seats were pocket boroughs in which there was factually no election. This continued until the Representation of the People Act in 1867 (a.k.a. the Reform Act).