The main difference between the United States and England (not UK) is that the U.S. has a federal constitution that sets out the construct of the nation and the central government. My understanding is that England has no written constitution which sets forth the relationship between the government and the government nor a plan for such. Instead, it simply has a series of unwritten understandings.
The power to govern in the U.S. is drawn from the people. (Not granted from the monarchy.) The people are organized into States. These States were originally the seats of real power with only specific powers granted to the Federal Government. As a matter of fact the U.S. constitution specifically states that if a power is not specifically granted within the body of the constitution the Federal government shall not have that power, but only the States shall. So in the theory of construct the U.S. is truly a United group of States under the written constitution. Each State governs its own territory. Thus different license plates.