Complex sentence: The City of Westminster, near London, had become the "de facto" capital of the Kingdom of England by the beginning of the 12th century, and later of the Kingdom of Great Britain (1707--1801).
Split into simpler sentences: The City of Westminster, near to London, had become the "de facto" capital by the beginning of the 12th century. It was the capital of the Kingdom of England, then of the Kingdom of Great Britain (1707--1801).