From Answers in Genesis
http://answersingenesis.org/docs2/4304news5-16-2000.asp

"...British Israelitism

Some have promoted the idea that the British people are descended from
the 10 allegedly 'lost tribes of Israel'. Alas, a major feature of
British Israelitism is that it ignores or explains away the weight of
biblical evidence, which must be normative for the Christian. It also
ignores the well-documented history of the Jews after the close of the
Biblical Canon, but places a heavy weight on non-biblical accounts of
dubious reliability. Fact is, the tribes were not lost. The Bible is
clear where the tribes went, and that representatives of all tribes
returned from Israel (Ezra 6:17). The prophetess Anna was a Jewess from
the tribe of Asher (Luke 2:36), which proves they were not 'lost' even
by the time of Christ. Even later, James wrote an epistle 'To the
twelve tribes scattered among the nations (literally 'To the twelve
tribes which are in the diaspora/dispersion')' James 1:1, and it would
be hard to know where to mail it if 10 of the tribes had been lost!

Some try to justify British Israelitism by spurious linguistic and
historic connection.8 For example, connecting the name 'Dan' with names
like Danube, Denmark, etc., without any historical records to demostrate
the connection. Another claim is that the tribe 'Saxons' is alleged to
come from Saac's Sons, a contraction of Isaac's Sons. Probably the most
common claim is that the word 'British' comes from berith ish,
supposedly meaning 'man of the covenant'.

But all this is linguistic nonsense. 'Saxon' is a Germanic word probably
coming from the name of the weapon they used, according to the Oxford
English Dictionary. It may come from the Latin saxum meaning 'rock'
(thus weapon made of rock), or from secare to cut. There is no link to
any non-Indo-European language. The alleged derivation from Isaac's
Sons is simply untenable-'Isaac' is the Greek form of the name (Isaak);
the original Hebrew form of the name is Yits-khaq (with four consonants:
yod, tsadhe, cheth, qoph).9 It boggles the mind that Israelite tribes
allegedly lost centuries before Christ would adopt a name that is a
contraction of a fairly modern English grammatical construction based
on the Greek form of the name of Israel's father! One must also wonder
why the same 'reasoning' wouldn't 'prove' a 'British Edomite' theory as
well, since Isaac was the father of Esau/Edom too. Also, what about the
modern Saxons in Saxony in Germany-surely they would have to be
Israelites as much as the British? And what about the Celtic Britons
(now Welsh and Irish) who lived before the Germanic Saxon invasions of
AD 450, or other non-Saxon British such as the Angles (after whom
England is named) and Jutes-do they count as Israelites too?

The correct Hebrew phrase for 'man of the covenant' is ish haberith.
Rather, 'British'/'Britain' comes from the Celtic tribe of the Britons
which inhabited England before any Germanic peoples, of which the
Saxons were only one, came. The form 'Britain' comes from the French
form of the Latin 'Britannia', which the Romans would have formed from
the name of the Celtic tribe.

And I never hear 'British Israelites' apply this etymological chicanery
consistently, because then they would have to admit that their own
reasoning leads to the word 'Britain' coming from berith 'ayin meaning
'without a covenant'.

Overall, British Israelitism seems to stem from ideas of European
superiority.

Bill Cooper's book After the Flood historically documents how many
European peoples, including the British (the Welsh, Saxons, etc.), are
descended from Japheth, while Jews come from Shem. And the study by
Hammer et al. show that the Jews are a Semitic people group, related
more closely to the Arabs than to non-Semitic people like the
British...."