England
Political Geography
First we need to establish the geography—what I mean when I say "England". By 1300, it was pretty close to the modern country. Wales, Scotland and Ireland were in varying degrees claimed by the English king, but control there was highly variable.
The English king laid claim to Wales by right of conquest. In fact, it was Edward I, still reigning in 1300, who established the practice of making his eldest son the "Prince of Wales" as a means of asserting his authority there. The Welsh were by no means reconciled to this and the early 1300s saw several serious revolts. By the 15th century, however, Wales was no longer a danger point.
Scotland was a chronic problem throughout the period. The Scots were in full-scale rebellion in the early 14th century. Once the Hundred Years War started, they entered into close relations with France, an alliance that proved extremely durable. The French often helped underwrite Scottish invasions of northern England, and the French court took in refugees when the English were able to gain the upper hand in Scotland.
Ireland was a land apart; the English claims here were hardly more than theoretical and enforced only when accompanied by an army. It rarely drained significant resources. Instead, during these centuries, it might from time to time, when there was a lull in other fighting, serve as a focus of activity. But when war flared up elsewhere, the troops went home, the Irish rebelled, and the English residents retired to their fortified towns and their castles.
Within England itself there were distinct regions. London and environs was so important that it was a force in and of itself. Kent and Sussex in the south were rich agricultural lands. At the other end of the country, on the Scottish border, was Northumbria, whose lords were often effectively independent of royal control and were sometimes in open opposition to the crown.
Although the great lords of England were titled earl or duke, with the latter title being imported from the Continent by Edward III. They typically had a center of power, some set of estates that were traditionally associated with the title, but their actual holdings tended to be widely scattered. This was the result of deliberate policy by English kings. Also, the king of England could reassign the title and reshuffle the lands pretty much at will, a level of control unknown in France or Germany. Some families tended to bear the title in certain areas (e.g., the Percies in Northumbria), but with most titles in England you will find that they change hands with almost every generation. This was even more noticeable in the civil wars of the 15th century.