Britain was no longer British after Anglo Saxon mercenaries mutinied against their Romano British paymasters. They wanted the land to expand into as their homelands in Europe were not enough for a growing population.
The Celts/Romano Britons, had withdrawn to the west (what is now Wales, Devon and Cornwall and a large portion of what is now the Midlands), the border between them and the Anglo Saxon ran from the Severn estuary in a sweeping curve up to Lothian.

The occupied areas had split into minor kingdoms with some paying dues to the larger, better-armed kingdoms.
The largest kingdoms were the Northumbrian; Mercia; East Angles and West Saxons. These eventually becoming Angle Land or England, and uniting the kingdoms (the Jute lands had already been absorbed by the greater strength of the Angles and the Saxons).
Rather than working together these early kingdoms not only fought against each other but also against a new wave of invaders from the North, often called Vikings but more often known as Danes.
By the late 9th century, 860 AD to 870 AD there had been battles between kingdoms for supremacy and battles against the invading Danes when two or more kingdoms might combine for strength.
Then came the Great Heathen Army (yes the various kingdoms had become Christians by now) in 865 which consisted of various armies from the Viking countries united to attack Anglo Saxon Britain.
The individual Anglo Saxon kingdoms were not overly keen to fight and the Mercian king was overthrown and it became a puppet kingdom.
The only real resistance came from Wessex and this is where the modern-day English (more a mixture of Anglo-Saxons, Norman, French, Scandinavian etc. etc.) gained one of their favourite heroes Alfred the Great.
Not that he was great at the time, he wasn’t even king of Wessex, that position was held by his brother Aethelread. Alfred did, however, lead the Wessex army and defeated the Danish army at Ashdown.
Alfred did become king three months later when his brother died. When the Danes turned up again Alfred did not face them in battle, instead he bought them off, just as other kingdoms had done.
The Danes were allowed to settle in the east, an area which became known as the Danegeld, as part of a deal which also saw them getting payments from the Anglo Saxons.
Alfred used this period to build up his own strength and create deals with the other Anglo Saxon groups to form a more cohesive group. This involved various deals and marriages until the time Alfred was accepted as King of all Anglo Saxons (which did not give him power of the lands held by the Danes).
This title (which marked Alfred as the first real king of England, was passed to his successor, his son Edward, when Alfred died in 899.
Although Mercia was still an entity in its own right it was subservient to the Kingdom of Wessex. A Northumbrian army (Danes) was then defeated by the Wessex/Mercian alliance and Edward turned to defeating other Danish enclaves in the southern region until only Northumbria was left to the Danes.
By now Edward had taken Mercia into Wessex and he was the Anglo Saxon ruler of all the lands from Northumbria down to the south coast and out to the east.
The line from Alfred the Great to Edward the Confessor (he’s the one who died and left no real heir but was succeeded by Harold Godwinson even though William of Normandy claimed he was the rightful heir) is fairly straightforward, although there was a glitch when a Danish king Cnut (we know him as Canute) ruled for a while.
Basically the title King of the Anglo Saxons (by the Confessor’s time King of England) went from Alfred to his son Edward the Elder (a lot of names were similar in those days which is why monarchs tended to have descriptions as well) and then to his son Aethelstan who called himself King of the English; he was succeeded by his half-brother (still a son of Edward) Edmund I, who was succeeded by his brother Eadred.
The crown then went to Eadred’s nephew (Edmund’s son) Eadwig the Fair and later to Eadwig’s brother, Edgar I the Peaceful and then to his son Edward the Martyr who was succeeded by his brother, Aethelred the Unready.
At which point we get the glitch and a temporary rule by the Danes who left England with an interesting tale about a throne on the beach and an incoming tide.
In 1012/13 Aethelred had been quite happily ruling England for over 30 years and the last thing he expected was for the Scandinavian hordes to descend on his kingdom.
It all began with Harald Bluetooth the Scandinavian king who gave his name to the modern Bluetooth communications system. He was usurped by Sweyn Forkbeard, who for centuries was believed to be his son. It appears Sweyn was actually the son of Harald’s brother Knut but was raised in Harald’s household.
Sweyn then headed an invasion fleet of England which landed in 1012 and his army was powerful enough to send Aethelred into exile and Sweyn ruled as King of England as well as with his Scandinavian titles.
He didn’t enjoy his new position for long as he died in 1014 and Aethelred came back to reclaim his throne. He didn’t have long either as two years later he died. He was still the longest-serving Anglo-Saxon king as his split sovereignty meant he ruled for 37 years.
His son Edmund Ironside replaced him but only lived a few months as king.
Cnut had remained in Scandinavia when his father died but he decided to take an invasion fleet to England and eventually forced Edmund to sign a deal in which the Viking would rule the area known as the Danegeld and would become king of England on the death of Edmund.
He didn’t have long to wait as Edmund died, probably of wounds received in battle, a few months later.
The foreign usurper actually proved to be a good king for 20 years but on his death the kingdom was returned to the hands of the Anglo Saxon dynasty when Edward the Confessor, another son of Aethelred, was eventually crowned king of England after a tussle between Cnut’s heirs and the Anglo Saxons. The Anglo Saxons won and brought Edward back from exile in Europe..
This gave peace in England for over 20 years until Edward died without an heir – but that’s another story.