I'm blessed in using the 17th Century as my baseline setting because there are so many different options for factions and groups that have reason to hate each other and pick fights with each other. As such, I think the PCs I run through the Once-Green and Pleasant Land are going to spend far more time fighting their fellow men then they will fighting demons...which is probably exactly what the demons want.
|even the dogs have taken sides.|
Cornishmen are the native inhabitants of Cornwall, descended from the Britonic people who dwelled there before the Roman Conquest. They maintain their own language (Cornish) and national identity; during the first half of the 17th century, efforts had been made to anglicize the Cornish, but these efforts have fallen apart in the wake of the Cataclysm and the Cornish have reasserted their unique identity.
Englishmen are the people of the remainder of England; descended from Normans, Anglo-Saxons and Romans, they're the dominant ethnic group in the British Isles and until the Cataclysm were making great headway in conquering and assimilating the other peoples of the Isles.
The Irish, native to Ireland, an island north of England, have been fighting a losing war for their national identity with the English; once great travelers and traders, the Irish saw the dawn of the 17th century with an invasion from England, and a second wave of reconquest in the 1640s under Oliver Cromwell. It is rumored that the atrocities committed by Cromwell on Irish soil are what caused the Cataclysm, and while the Irish have largely reasserted their sovereignty in its wake, their hatred of the English continues to burn.
The Welsh, native to Wales, are closely related to the Cornish, and speak their own, related language predating the Saxons and Norman invaders of the Dark and Middle Ages. They're more closely tied politically to the English then the Cornish are, but in the wake of the Cataclysm a shift away from England is still being observed.
The Scottish, from Scotland in the north of England, were initially supporters of the Royalists during the War, for both religious and political reasons, and were also involved in an internal civil war between their own Royalist and Parliamentarian (called Covenanter) factions. When the alliance between Coventers and English Parliamentarians broke down, Cromwell led an invading army into Scotland to settle their shit. In the wake of the Cataclysm, various warbands of Scottish Reivers roam the Once-Green and Pleasant Land as mercenaries and brigands.
Invaders: Free companies of enterprising Swedish and Dutch adventurers probably occasionally make landfall in the Once-Green and Pleasant Land looking for plunder, as would German Landsknechts left without gainful employment following the Thirty Years War. France, weakened by the revolution of the Fronde and the Franco-Spanish War, probably aren't too terribly interested in England though French pirates operating out of Brittany would likely be harrying ports in Cornwall for tin and ceramics.
Royalists, or Cavaliers, are those who support the monarchy of Charles I (until his January 1649 execution), then Charles II (reign: February 1649 to April 1651 AD/April 2 AP). Now most Cavaliers support one of any number of claimants to the vacant throne of England, most of them claiming some blood relation to the Stuart or even the Tudor dynasty, or otherwise claiming especial favor of one of the deceased kings. They tend to dress in high leather boots and ostentatious hats, and have taken well to the new "traditional" means of warfare engendered by the scarcity of gunpowder in the Once-Green and Pleasant Land.
Parliamentarians, or Roundheads, are those who pushed for a constitutional monarchy in England as opposed to the absolutist monarchy of Charles I, or even complete parliamentary control of executive government. In the wake of the Cataclysm, many Roundheads (so named for their close-cropped hair, as opposed to the courtly style of long ringlets favored by the Cavaliers) have taken to organizing themselves into semi-egalitarian warbands that harass and terrorize any groups of Cavaliers they can find, regardless of which "king" they support.
I think I kinda wanna make random charts for players to roll on to determine ethnic group and political affiliation at character creation. I think it would make for an interesting dynamic to have, say, a Cornish Royalist Fighter, a Scottish Parliamentarian Cleric, a Welsh Specialist of no particular political affiliation and an English Royalist Magic-User in the same party.