I am not a christian but this is one incredible theory with lots of evidence. It basically says the 12 to 13 or so lost tribes of Israel were taken into captivity and slavery by the Assyrians who migrated to what is now Germany and took their Israelite slaves with them. Eventually the Israelite slaves were allowed to migrate throughout Northwestern Europe including the British Isles when the Assyrian Empire began to collapse (though many stayed in Germany to). Here is where I read about it and read it for yourself here
AMERICA and BRITAIN in Bible Prophecy