The main problem of course is the Saxons... oh, and the Angles.
I didn't mind the Jutes or the Danes too much, they integrated well with us native Celts, but these Anglo-Saxon types refuse to worship our gods, eat funny food and generally look different. Send them all home I say!
Judging by some views expressed on this topic, I think the main issue was when Homo Sapiens immigrated here from Europe and displaced Homo Heidelbergensis who, at the time of the wave of immigration, were living in Sussex minding their own business. "All those Sapiens', coming here with their bigger brains. You mark my words, they'll destroy our culture as we know it".