reader.
Wikipedia References:

History of the British Isles
English people
Anglo-Saxon settlement of Britain
History of Wales
History of Scotland

A:

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 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 notbetween 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.

A:

The obvious reason for Scotland being "conquered" by England is that King James VI of Scotland was heir to the English throne, and upon the death of Elizabeth I of England (and Ireland) found himself ruling both kingdoms.  The larger English population and stronger economy then led to the English language gradually pushing aside both Scottish Gaelic and Scots (a Germanic tongue descended from Old English).
However, the legal system of Scotland is distinct from that of England, and the  establishment of the Scottish Assembly promises to retain that distinction.

