Thanks for your question. In brief, this topic -- which has fascinated some people for decades -- is not covered directly (in the way one might suppose) in Scripture. Some people have assumed wrongly that the "lost tribes of Israel" (there actually is no such thing, theologically speaking!) formed the British people, and other such odd theories abound.
Let us not be fanciful. Scripture is our sole authority for religious truth, and we must limit ourselves to what Scripture says about various issues -- irrespective of our curiosity.
Under King Solomon's reign, rebellious Israel was divided into Northern and Southern (Judah) Kingdoms. Most of ethnic Israel had abandoned the Lord and assimilated the religious and cultural dictums of the surrounding pagans: thus the Lord judged His people, and only a faithful remnant remained to re-enter the land. We might call these people "spiritual Israel," in the same sense that any believer in the Lord Jesus Christ today is a member of spiritual Israel (God's one true people).
Those Jews who did not repent and follow the Lord were dispersed among the nations, and their descendants are among us today. Ethnic distinctions don't matter to God anymore, and so the descendants of the unfaithful in the 12 tribes of Israel are no different from the descendants of the unbelieving Gentiles of yesteryear.
We would be wise to recall the words of the inspired Apostle John at this point: "He who hath the Son hath life; he who hath not the Son hath not life." Therein lies the most-important distinction in human history: between believers and non-believers.