When did England become a country?

Well, the question leaves many options...

The Celts and other nomadic tribes settled the lower British Isles about 4000-5000 years ago, although they wouldn't have called it England.

The Romans, under Julius Caesar, invaded around 55 BC, stayed for a while, started a city called Londinium on the Thames, but left the tribes before the fall of Rome.

After that, the native tribes and/or the Anglo-Saxon invaders formed seven kingdoms (East Anglia, Northumbria, Kent, Mercia, Essex, Sussex, Wessex) and by 802 AD Egbert, King of Wessex, was accepted as King of the English.

But most people would believe that England really became itself with the Norman conquest of 1066.