The myth of the "Ten Lost Tribes" is the basis for "British- Israelism" and other colorful legends, but these stories have no real Biblical basis. They are based upon misconceptions derived from the misreading of various Bible passages. (2 Kings 17:7-23; 2 Chronicles 6:6-11)
When the Northern Kingdom went into captivity (722 b.c.), all 12 tribes were also represented in the south. When the Babylonians took the Southern Kingdom into captivity (586 b.c.), members of all 12 tribes of Israel were involved. Isaiah, prophesying to Judah, refers to them as the "House of Jacob, which are called by the name of Israel..." (Isaiah 48:1; cf. vv. 12-14).
Accompanying some of the legends of the so-called "Ten Lost Tribes" are aspersions on the present State of Israel and the people being regathered in the Land. These various theories such as "British Israelism" are by their nature anti-Semitic because they deny the Jewish people their proper place in the plan of God. Let's remember that Genesis 12:3 has never been repealed!
Sources: Mystery of the Myth: The Ten Lost Tribes by Chuck Missler
http://www.khouse.org/articles/1995/40/