The Romans did not leave any great impression ethnically or culturally on England. The Romans who came over became an aristocracy and many left when the Romans pulled out in 410. The Celts who lived in England were either forced out or fled into Wales, Cornwall and Strathclyde. Their ethnical impact on modern England was not very large, their cultural influence much smaller.The Anglo-Saxons left the greatest, most important impression on England. Most people in England are part or majoratily Anglo Saxon and they left a great mark on the culture, through the language and other things. The Vikings left a greater mark ethnically on the Danelaw section of England, certain parts of Wales, Scotland and Ireland, the Isle of Man and the Orkney, Hebrides and the Shetland Islands. Their cultural impact is much harder to see, since they came from a similar Germanic culture as the Anglo-Saxons. The Normans formed an aristocracy, so left little ethnic influence, but the mix of Old English and Norman French resulted in the birth of modern English.

Coming back to topic, I think it was a civil war as they were many colonists who fought for the Crown. 20-30% of the White population and a good deal more of the slave and Red Indian population supported the British. Around 100,000 loyalists- about four percent of the population left the US at the end of the war, leaving for Canada and Britian.