Although I agree with almost all of what you say Lizzie, there's one thing that you don't say (forgive me if you have but I've missed it).
That is, the population that is 'invaded' by newcomers at all these points in history were (I speculate!) never very happy about what was happening at the time.
Just because mankind theoretically has the capacity to study and learn from history, to potentially remove the fear aspect by seeing that 'things turned out OK last time', doesn't mean people will think that way. They'll just be afraid and cross, mostly.
Absolutely right, they were not at the time. 
However, within a short space of time the Romans, the Anglo Saxons, the Vikings and the Normans , who caused the biggest changes before the birth of the British Empire, were considered of great benefit to those living in England, and were missed when their power faded.
To cut a long, long story: Leaping forward in time with the coming of the Blacks from former / existing British colonies in the 1950's were not welcomed. But now how we value their descendants who are medical, legal, engineering, scientific, teaching, etc, etc, professionals, and how we could not do without them!
It is especially the English who have NEVER welcomed change, especially if it was not white Christian English, but that did not include Catholics or Protestants at certain periods in our history, and as for the Welsh of Scottish peoples..............well jokes still abound in English circles about those tribes who once were enemies of England but now are really close friends.
We soon get over change, and that drove the Empire and the Industrial Revolution on to achieve really Great things, often with the aid of "foreigners" who found their way to our fair shores.
Finally, I would ask can anyone in this Great Isle really claim a PURE British blood line, with truely British DNA all the way back to the ancient Brits? They are the rarity, not us who are in the huge majority from IMMIGRANT blood and DNA. The mix has been the wonderful factor in British history and has avoided the terrible inter breeding that otherwise would have taken place, as it did in certain sectors of the aristocracy!

Here is a DT article on some of their trades that you missed off your list:
http://www.telegraph.co.uk/news/uknews/crime/9840073/Revealed-gang-rivalry-Mark-Duggan-and-the-unavenged-murder-behind-the-London-riots.html
What does that prove Rods2?

There is good and bad in all, no matter of what colour, creed, culture, or religion. Just think how many very serious crimes are committed by whites, but that is in proportion with the population who are white. That is the same with blacks. However, when a certain race is isolated by the majority, and given the hard shoulder, then a feeling of hitting back may increase their level of crime!
Remember, Hitler and Stalin were white! So was Guy Fawkes, Dr. Crippen, Jack the Ripper, the Yorkshire Ripper, The Great Train Robbers, Raoul Moat, and 100s of thousands of other serious criminals, oh, including Fagin (yes there were many Jewish Fagin's in Victorian England)!
You can always put a colour to a crime and paint all with the same brush.
Anyway, for all those I cannot convince with my numerous posts, let me leave you with this thought:
Organise a film unit, get a brilliant director, a bunch of evil looking foreigners - maybe Muslims - then make a film of their faces in dark sinister tones, with evil threatening looks. Edit the film of shots with rats, or snakes, or spiders - something the majority of people fear - and blend in with the ghastly, horrible, wicked foreigners, and put a commentary on showing how they threaten the country. Then show the completed film everywhere to white sympathetic audiences, ensuring you transmit the message that all these dreadful foreigners businesses are boycotted, even attacked, with your mobs roaming the streets to rough up, even kill these dirty foreigners. Try even to get Parliament to pass an act making all of these disgusting people aliens, and criminals of the State, then when passed organise a "Black Knight" night to round up and ruin all these undesirables, to finally rid the State of them.
After that you will remember that your DNA includes elements from foreign, non-English, descent. Then the State machine will come for you!!
ENOUGH!!! I am making myself sick writing this script. But I am sure you will remember and consider what history has taught us and how humans can quickly turn into beasts to consume each other when not conforming to their ideas of normality!
