The Kingdom of England was formally unified in 1066 after the Norman Conquest that ended the factious ideals of the Saxon monarchs (OTL). Its first King was William I, who introduced a feudal system similar to that in France - and supervised the construction of castles to bring the local saxons in line with his monarchy. Over time, this Norman dominated English state expanded until, in the 16th century, it became known as the British Empire.
The Issue of Normandy
William I ruled both the Kingdom of England and the Duchy of Normandy until his death in 1087. His domains were then divided between Robert, who got Normandy, and William Rufus (William II) who got England. Due to an absence of Muslims in the Holy Land (and a Byzantine presence too strong to be shifted), Robert of Normandy did not go crusading - and consequently did not pawn his Duchy to his erstwhile brother and was allowed to make it into a fully functional state once again. Rufus, on his part, did little to challenge his brother and died in 1100 from a riding accident. He was succeeded by his brother, Henry.
Henry, who had felt a little hard done by from his initial inheritance (a minimal payment from the royal coffers) decided to dispute the rule of Normandy with his brother, Robert. However, due to Roberts unhindered presence in his duchy, battle honours were equally divided. Henry may have achieved victory at Tinchebrai in 1106 - and seized the towns of Bayeux and Caen, but Robert was able to recover from these losses and strike out from his new base at Le Havre. Henry's army was brought to bay at the battle of Caen and reduced to such numbers that Henry was forced to withdraw from Normandy. Robert, however, had also suffered crippling losses at the same battle and was forced to buy peace from the English king. Moreover, serious concessions were made to the King of France regarding the role of Normandy in France's feudal system. Normandy was no longer the independent nation it had been - and was no more concern to Henry.
The Death of Henry I
Henry ruled diligently until his death in 1135, but due to the tragic death of his son, William, in the White Ship Tragedy, succession crisis was caused when Henry's daughter Matilda, former wife of the Holy Roman Emperor and now married to the Duke of Anjou, was deemed unfit to rule by England's most powerful magnates. They instead chose Stephen, the feeble nephew of the dead king, as their champion. However, the indomitable Duke of Normandy, Robert, quashed in his attempts to assert himself on the mainland by the intelligent dealings of France's Capetian monarchs, decided to throw his claim into the mix. Stephen and Matilda's war was hastily joined and though Robert attained a few successes, he was dead by 1144. His son, William, wisely decided not to interfere.
And so the succession ultimately passed to Matilda's son Henry of Anjou - on Stephen's death in 1154. Henry's marriage to Eleanor of Aquitaine gave a line of English possessions all along the French Atlantic coast, which more than once frightened feeble Louix VIII into submission. His passing in 1180 gave way to a far more capable monarch; his son Philip II Augustus. This monarch was all to capable of coaxing his warlike vassals, namely the Duke of Normandy and the Spanish Marcher Lords, into defending his interests against the English - and also knew how to play off Henry II's sons against him. Though Henry staved all assaults magnificently - including one in 1173 launched by his namesake son - the strain broke both him and his resources. By 1189, he was dead, replaced by his son, Richard I.