The book was put together using the best research available at the time, but when your starting with the Trojans founding the British nation and going all the way until the seventh century when the Anglo-Saxons took over, you're going to have to fill in some gaps.
If you take a close look, there is definitely some historical facts, such as the invasion of Julius Caeser, but the events surrounding those events are not even close to being accurate. Still, the book remains an important part of English literature, which was the class where I first read it.
You'll find that the heroes of English monarchy had a lot of help from the gods and more than a little magic. This makes for an entertaining read to say the least. Whether it's Aeneas's grandson discovering the land after being banished from Troy and guided by Diana or Merlin helping a young King Arthur.
Much of the legend of King Arthur from the help of Merlin to his wounding my Mordred are covered in the book and has been an inspiration to authors centuries afterward. If you are looking for a great piece of classical English literature, then you will find nothing better than “History of the Kings of Briton.” If you are looking for a factual history about England, then look somewhere else.