England as a unified kingdom was not formed in 1066 - that's the date of the Norman Conquest, which has traditionally been taught in schools as if it were the beginning of English history, so I can see where that misconception comes from. The Anglo-Saxon king Athelstan, who reigned about 150 years before the Conquest, is usually credited as the first king of a unified England.

The kingdom of Mercia is probably what you're thinking of, which covered what we now call the midlands. Mercia was the dominant power among the Anglo-Saxon kingdoms during the 8th century. Its king Offa exchanged letters with Charlemagne, and left his mark on the countryside by constructing his famous Dyke along the border with Wales, which can still be seen today. Most of Mercia fell into Danish hands during the Viking invasions of the ninth century, and after England was unified by the West Saxon kings it never regained its independence.