So, how old is Britain anyway?

In America we have a birthdate for the United States. It's when we offically thumbed our nose at you guys. But who did you guys thumb your noses at? The Romans? The French? Actually it was kind of the other way around. The took Caratacus back home and paraded him through the streets in a Triumph. The French arrived (as Normans) and taught you a new language. Which you successfully absorbed in one of the great turnabout conquests of all time. (Good job on that, by the way!)

So, whence do y'all date from?