England lies in the warmest, richest, and most fertile parts of the British Isles. These are modern population figures, but they are indicative of past relative strengths:
England, 55 million; Ireland (counting northern Ireland), 6 million; Scotland, 5 million, Wales, 3 million. Frankly, I was surprised at the disparity between England, and all others (14 million). This is in spite of the fact that England has slightly less land area than the others put together. England also has (by far)had the highest per capital GDP of the four until modern times when an influx of foreign capital enabled the other regions to catch up to, or even overtake England.
So historically, a lot of the competition was not between England and the others, but rather between various "English" factions; e.g. Alfred and Guthrum, or William the Conqueror and Harold. Put another way, the competition was between "British" factions, of whom only some were non-English.