Two replies: 1) The more serious, regarding the DNA makeup of the British population being largely unaltered by migration, this is precisely the point made by Stephen Oppenheimer in his book "The origins of the British", now available in paperback,his thesis is that the modern population of Europe originated in "Refuges" from the last Ice Age. The one most affecting us being situated in what is now the Basque area of Northeastern Spain, the others situated in the Alpine region of Italy/Austria, and what is now the Ukraine had less effect.
That theory is several years out of date (like his book) now.
The theory that has replaced it is that the early N Europeans that hunted wooly mammoths, lived alongside Neanderthals and were the first to settle Britain, retreated as you say to the "Refuges" in the last ice age and then repopulated N Europe. They were largely made of YDNA Haplogroup I and mtDNA Haplogroup U.
However, c 5000 yrs ago there was a large influx of horse-riding, charioteering, farming peoples from the Black Sea (and Asia before that). They moved through the Anatolia (Turkey) and S Balkans areas into central Europe around Germany/Switzerland. Their population exploded and they expanded rapidly N, E, S & W mainly in the late bronze & iron age. As they overran areas where the early hunter gatherers lived it is thought that (as is usual with invasions) they slaughtered the men and stole their women. They were largely comprised of Y Haplogroup R1 and mtDNA Haplogroup H + a few others.
I represent pretty much the average result of this theory, ie. I am R1b on my paternal side so am descended from the "recent" bronze/iron age immigrants but am also mtDNA HG U5 which means my maternal forebear was one of the "ancient" hunter gatherers who hung out with the Neanderthals around Finland or there abouts. In effect a cross between Old and New European.
I have probably explained it very badly but I think this site might enlighten/confuse you more:-
http://www.eupedia.com/europe/origins_haplogroups_europe.shtml
