Really Ancient
Islands are prone to invasion.  In Neolithic times, folk walked across the then-joined landmass of ancient France to England.  These Iberians, as they are called, were later joined by - amongst others - the Celts (originally occupying northwest Germany and the Netherlands) and the Picts.  The Brythons or Britons (who gave their name to Britain) arrived next; a remnant of their speech lives on in the tongues of Wales, Cornwall, and Brittany.

Ancient
Julius Cæsar launched two expeditions against the island in 55 and 54 BCE. The campagin in 54 was quite a major affair, with eight hundred ships and 27,000 troops, but essentially yielded little more than tribute and a few hostages. Cæsar never returned to Britain again. Claudius built on and extended the Roman holdings beginning in 43 CE - he even brought his war elephants with him. The Romans were serious about Britain, establishing cities, building magnificent villas, criss-crossing the land with their stone roads, still to be seen today. The native "Britons" were subdued or ignored, but not annihilated.  Many became Romanised, adopting Roman laws, dress, custom, and religion - first Roman paganism in all its many forms, and then Christianity.

Almost Modern
By the early 5th century however, the Roman legions stationed in England were required nearer to home as the Empire was increasingly threatened.  The Romanised Britons were left to their own defences. They were not up to the task. 

Raiding heathen pirates - Jutes, Saxons, and Angles - began visiting the shores, beginning with the brothers Hengist and Horsa in 449.  (These three tribes came from what is now coastal Belgium, Netherlands, northern Germany and Denmark.)  Note that Hengist and Horsa had originally been hired by the Britons to fight off the Picts invading from the North (Scotland), and then turned on their employers. 

Successive war bands, intent not only on treasure but on settlement, followed.  They found little real resistance, save possibly in one Dux Bellorum (general of the British army), named Arthur.  This resistance was temporary, however, as wide-scale immigration continued, including the populations of entire Saxon villages which crossed over to the new land to settle. 

The newcomers cared nothing for the glories and pleasure of Roman civilisation, destroying and abandoning Roman cities and taking up a pastoral existence of hunting and farming. 

The Angles and Saxons continued to thrive in England (Angle-land).  Society was arranged around powerful war-lords, who with their sworn warriors, defended and increased their local holdings.  The most powerful war-lords became kings of rapidly-changing kingdoms.  Typically throughout this era, the larger portion of England would be divided into four to seven individual kingdoms.  Kingship was not hereditary; the strongest, richest warrior was generally acknowledged as king, and kingdoms changed hands violently, and with startling frequency. 

In 597 Christianity arrived again in England when Augustine, sent by Pope Gregory, landed at the isle of Thanet in Kent and was given permission to preach by King Æthelbert.  (The Celtic Christian church in Britain had never ceased to operate, but was confined to remote monasteries in Ireland and the North.) 

The Christian conversion continued in fits and starts.  If Church records are to be believed - and we have very little other records to rely upon - within 200 years most Anglo-Saxons and remaining Britons (the Welsh, etc.) were Christian. 

The Danes, repeating the actions of the Anglo-Saxons 400 years earlier, began invading England in earnest in 865.  This brings us to 871, when a young, scholarly, and sickly Ælfred became king of Wessex, the year in which The Circle of Ceridwen is set. 

You now know more than enough to pass any exam on the topic.