The Isle is divided into three main areas:
the Kingdoms of Aengland,
the Kingdoms of Cymri and
the Kingdoms of Alba.

Most of the population considers itself "Aenglish," though descended from the Angles, Saxons, Jutes, or Frisians. A fair portion, however, especially in the western areas, are actually of Brittanic ancestry. Some interbreeding has resulted both in people considered to be Aenglish, but who are somewhat Brittanic, and vice-versa. Many shires are full of Brittanic folk who are ruled by the Aenglish overclass.

Northumbria
North of the River Humber, and mostly east of the mountains. Northumbria has been having the most trouble with raiding Norsemen. The Aenglish folk here are less frequently Christian, and more likely to be Wotanists.

Mercia
The heart of the Aenglish kingdoms, Mercia is the largest. Almost everyone is Christian, including most of the subject Brittanic peoples.

Wessex
A fairly small saxon kingdom, but one with much influence and gaining power. Wessex occupies the outhwest of the island, and including most of Cornwall, the western part of which still retains its freedom violently.

East Anglia, Essex, Sussex, and Kent
There's not much I can say about these small kingdoms in the southeast of the island. They'll probably get absorbed into Mercia before too much longer. The East Anglians are Algles, The people of Essex and Sussex are Saxon, and the Kentish are mostly Frisian. No, I don't really know what that means either.

The western kingdoms of the Brittanic people were still monarchies even when the Romans had worked their influence over the eastern states. They remain fiercely monarchist now, and even more hostile to the Aenglish than to the Romans.

Gwenned, Powys, Ceredidgion, Glywysing, and Dyffed
These happen to be the current names of the kingdoms. At any given time, however, due to the long and convoluted political history of the region, the kings reserve the right to conquer, annex, divide, or inherit each other's lands. The Aenglish call these people "Welsh" for reasons really only clear to them. The kingdom of Dyffed, just to liven things up, is actually mostly filled with settlers from the Isle of Eire.

Strathclyde Region (Gallgaedhel and Cumbria)
Still on the west coast, but further to the north and separated from the above kingdoms by the Aenglish, there lie more lands of native Brittanic tribes. Keeping these lands soverign is an advantage to Northumbria; this way, the Norsemen attacking the western fjords of the Isle are somebody else's problem.

Even more savage (from the Aenglish point of view) than the "Welsh" are the short, blue-painted weirdos who spend all their time fighting, herding, farming, and conducting mercantilism and administration. What a bunch of backward third-world hicks.

Fib, Fortriu, Circind, Ce, Fidach, and Cath
These are the rumored names of the Kingdoms of Alba, thought the Aenglish aren't quite sure about that. Only the Eclesiasty from Northumbria and sometimes Rome go there. They seem to have decided that this is a good idea. They seem to be Green-Eggs-And_Ham-ishly convinced that if they go on about The Lord long enough, the Pechtii will eat him in a box. So to speak. Maybe it's some form of penance.

Dal Riada
In the southwest of Alba is a kingdom inhabited by the Scotti from the Isle of Eire. They insist that they are entirely unlike the southern Eirish in Dyffed, though the Aenglish would be hard pressed to see any difference. They like to wear their tartan cloaks pleated and belted around their long tunics, making a kind of kilt-folded skirt. Who knows, maybe it'll catch on.