It’s mostly a matter of physical geography. Take a look at this satellite image of the British Isles, without any borders.
Right away you can see where the hilly and mountainous terrain is – mostly in Great Britain’s north and west. Mountainous terrain gives you benefits as well as drawbacks:
The rugged land is more difficult to occupy and invade, meaning that their distinctive cultures survived longer as independent nations, in the face of Saxon and Norman expansion. By the time the Kingdom of Great Britain came about in 1706, Wales and Scotland were the last bits of Great Britain that hadn’t been fully Anglicized.
However, that same rugged land is less productive. Transportation is more difficult, climate is harsher and soil is less fertile. This means that the lowlands of England win out when it comes to supporting a large population.