As this review in The Scotsman points out:
What does the story of Britain tell us about today's debate about identity?
For Miles, Britain's history demonstrates the viability of a multicultural society and the importance of continued mass immigration. For Nick Griffin, of the British National
(racist: ed.) Party , on the other hand, Miles's research demonstrates that Britain is "not a nation of immigrants", but "a nation of indigenous peoples descended from only a few hundred people who have lived on our homeland for tens of thousands of years" and that "multiculturalism and mass immigration are genocidal crimes against our people and our land".
It's not the facts of history that shape our sense of identity but the politics of identity that all too often shape our view of history.
http://thescotsman.scotsman.com/critique.cfm?id=1635532005And the trouble with that "80%" figure is that it obscures the differences inside Britain:
This is where the new techniques of genetic analysis come into their own. When researchers recovered DNA from the tooth of an Ice Age huntsman buried in a cave in Cheddar Gorge, they found that it matched the DNA of the history master, and two of his pupils, at the local school. No doubt those Somerset folk had also acquired, in the intervening 9,000 years, many ancestors whose origins lay elsewhere (Celts, Romans, Saxons, Huguenots, and so on). But the underlying continuity is a striking fact, all the more striking because it was unknowable until a few years ago.
Unfortunately, though, individual findings such as these are much more clear-cut than any overall pattern can be. Miles notes that there is a sort of Celtic-Germanic gradient running west-east across Britain: the further east an English family comes from, the more likely it is to share its DNA with people from the Netherlands, Denmark and Germany. But, he suggests, this could be the result of long-term interactions on a small scale; it does not necessarily imply any single episode of mass-migration.
http://arts.telegraph.co.uk/arts/main.jhtml?xml=/arts/2005/07/10/bomil10.xml(Somerset in on the west coast of England). Which means the "invading hordes from the North Sea, who pushed the Celts into Wales and bits of Scotland and Cornwall" theory (which was standardly taught in school, before DNA analysis) isn't completely wrong, it just paints a picture with sharper dividing lines than reality.