Genetic origin
The most important reference for this chapter is Stephen
Oppenheimer's book "The Origins of the British (a genetic detective
story)".
Half the book discusses the genetic background of the British
and the link with the Continent, complete with charts and statistics.
The other half consists of other arguments, such as language problems,
the real origin of the Celts, etcetera. Oppenheimer came to the
conclusion that the Anglo-Saxons probably did not import English into
Britain. Next to genetic findings, he uses virtually the same arguments
as in this website [1]. The main genetic reason ? The
Anglo-Saxons were not numerous enough. There was no massive migration
into Britain. Here is perhaps one of the most interesting quotes: (page
381):
Perhaps the best validation of my matching approach to specific
gene flow into Britain from southern Scandinavia and the Cimbrian
Peninsula [Denmark] is the nearly complete absence from the British
Isles of the numerous gene types specific to Frisia. Frisia is so much
nearer geographically to eastern Britain, and so much closer in
language and gene-group mix, that, on the basis of neighbourly
affinities it would be expected to have more valid matches than the
Cimbrian Peninsula [2] - but it has virtually none.
And there was no wipeout in Britain.
Matched Danish gene types in the British Isles, although to
some extend overlapping, also differ sharply from the 'Anglo-Saxon'
ones in that they are found both within and outside England in a
characteristic coastal distribution, geographically suggestive of
historically recorded Viking raids. Their different distribution but
similar approximate dating suggest that the Vikings are more likely
culprits than the Angles.."
Genetic relationships in Europe

Sorry for the quality of this reproduction. The gray zone
represents the British Isles. 'Ruisko' is the link with the Basque, and
is on the left. 'Ivan' (Serbia) and 'Rostov' (Ukraine) are other Ice
Age refuge markers, on the right. Notice that the Dutch, Germans and
Scandinavians are closer to 'Rostov' and 'Ivan', and remote from the
British. The Belgians are however in the middle of the British zone
which is closer to 'Ruisko', the Basque genetic marker. The French are
on the border, the Fries just outside. The more 'Welsh', the more they
are found on the left hand side.
About the presence of the Celts in Britain, Oppenheimer came to this
conclusion (page 409):
Gildas's Dark Ages horror story of the Saxon invasion has
generated the view that Celts were somehow the aboriginal
population of the British Isles. The view that languages in Britain
were 100% celtic when the Romans invaded is part of this false
assumption. If 'aboriginal' means Neolithic immigrants speaking celtic
languages who had replaced the former inhabitants of Britain, then
nothing could be further from the truth. The genetic evidence does not
support this at all. There was no Celtic replacement, any more than
there was an Anglo-Saxon replacement.
Clearly, both British peoples were already there just after
the LGM (Last Glacial Maximum) (page 128):
"My re-analysis confirms the general trend of similarity
across the North Sea. However, there are older reasons - and evidence-
for this genetic neighbourliness than a massive swamping of England
during the Dark Ages. The drowned North Sea Plain is one of the oldest
geographical indicators of the beginning of an eastern British
identity, and there is genetic evidence to support this. "
Oppenheimer confirms the
presence of a Germanic language in England and a 'celtic' language in
proto-Wales. As I will explain later, all began when the North Sea
filled up.
The real origin of the British lies in northern
Spain, especially for the 'west-Britons'. I feel however that the
presence of the Brythonic language in West-Britain causes a problem.
Why is Basque not spoken in West-Britain? Maybe there was 'once
upon a pre-Celtic time' a substantial language change in
West-Britain. The Basques could have been less adapted to the cold in
the winter, forced to shelter in their southern homeland and so their
coastal territories might be invaded by inland people. Or, quite
simply, they preferred to pass the winter in the warm south, with
family.