I think that most western countries have assimilated other cultures into their land, including UK. The fact that the national dish of England is not fish and chips but rather curry suggests a strong melding of some cultures. However, the majority of the influx in places like England is limited in comparison to USA.
Ok dear point noted- USA is at least 4 times the size of 'lil England, granted!;)
There is a larger mix of communities here and the interaction between cultures is even more. Another historical fact leaning towards USA is that America is historically a nation of non Americans. The British first came across in the 1600s and were soon followed by other international communities. So no American is really a true American whereas in England, you will still be able to find Caucasians who have a long heritage of British blood.
Actually youre not entirely correct there- the migration of land warfaring people came from Europe - Scandanavia- the Celts, Angles, Danes, Saxons and the French- they reproduced with the then hunters/ gathers tribesclansmen... who were a mish mash of Goths and Huns and they evolved in what we have today - A Brit. As youre alos ware our Royal Monarchy and family are in nowhere " REal Brits" there bloodline can be traced to the Germans- Sax Coburg- they changed their name due to name associations wit the Germans durin g the Great War was unfavourable with public opinion....