This question should be traced back to the 3rd ice age,there were people lived in Briton island.The first known settlers of Britain were the Iberians.They came from the Europe Continent and settled down at the southeast of Briton Island.About 700 B.C.,the Celts from the west of Europe started to move in these islands,one of them names Britons.At that time,the Celts had already know the usage of iron,the technology of plough was continuing develop,and the began to use currency.The productivity development promotes the differential of the Celts society.
Then it comes the Rome Invasion.During the Gallic Wars,Julius Caesar invaded Britain twice,in 55 and 54 B.C.The first invasion was made in summer,it was unsuccessful,but it paved the way for the second one which was happened in 43 A.D.the Roman emperor Claudiu started another incasion in Britain.This time he succeed.The Romans ruled Brtian for almost 300 years.
In 408 A.D.the roman has to withdrawl from this island,it was owned by the people from Northern Europe,They were know as the Gemantic people,it divided into three tribes,Angles,Saxons and Jutes.The Celts became slaves and many of them were killed.
By 850 AD,the country was under sustained attacks from Viking raids.This is the Denmark invasion.After the death of the emperor's grandson,the Anglo-Saxon people gain the country back,but by the 11th century,the Vikings came back again.this period the emperor is Harold.after his death,there's no strong English successor
Norman Conquest:in 1066 took place on the last successful invasion of England.Duke William of Mormandy in the battle of Hasting defeated the British,the Normans from France and others come here to settle.In the ensuing three centuries,the French became the language of nobility,the legal structure has been the English Channel on the other side of the passage of that set of consequences,and social structure,to some extent been affected.
Above all are the mailnly invasion history of this country,hopes it helpful.I have some more in detail if you need.