}Are you a British Israelist?
No, but what if I was? Just what is your hang up anyway?
The most elementary reading of Biblical history shows that Jews did not descend from the 10 Tribes of Israel which made up the so called Northern Kingdom. (They descended from the so called Southern Kingdom.) There is also no Biblical or other history to suggest those tribes either stayed there or assimilated with the Assyrians, as popular history (A division of Popular Mechanics) and uninformed Biblical fiction suggests.
Now tell us, just WHAT IS your agenda?
You might want to take a look at this scripture.
The Levites left the kingdom of Israel for Jerusalem.
And the priests and the Levites that were in all Israel resorted to him from all places where they lived.
For the Levites left their common lands and their holdings and came to Judah and Jerusalem, because Jerobo'am and his sons cast them out from serving as priests of the LORD (2 Chronicles 11:13-14)
And we also see that so did members of all the other tribes.
And those who had set their hearts to seek the LORD God of Israel came after them from all the tribes of Israel to Jerusalem to sacrifice to the LORD, the God of their fathers.
They strengthened the kingdom of Judah, and for three years they made Rehobo'am the son of Solomon secure, for they walked for three years in the way of David and Solomon. (2 Chronicles 11:16-17)
Over a period of time, more people from the northern kingdom migrated south to Judah.
When Asa heard these words, the prophecy of Azari'ah the son of Oded, he took courage, and put away the abominable idols from all the land of Judah and Benjamin and from the cities which he had taken in the hill country of E'phraim, and he repaired the altar of the LORD that was in front of the vestibule of the house of the LORD.
And he gathered all Judah and Benjamin, and those from E'phraim, Manas'seh, and Simeon who were sojourning with them, for great numbers had deserted to him from Israel when they saw that the LORD his God was with him.
They were gathered at Jerusalem in the third month of the fifteenth year of the reign of Asa. (2 Chronicles 15:8-10)
Corroborating these scriptural accounts, archaeological evidence shows a substantial increase in the population of Judah at this time.
Bottom line is, while the southern kingdom was predominantly of the tribe of Judah, there were people represented there from all twelve tribes. And therefore it is from all twelve tribes that modern Jews are descended.
This is not to say that other peoples are not also descended from the 10 northern tribes. But it does refute the notion that modern Jews are only from the tribe of Judah.