Why have I been removed from the uk leaderboard of top contributrors?

The 10th person on the UK leaderboard has 383,833 points. As you have a great many less than that there is no way you could have been in the top 10.

Are you asking about the TC badge/crown, or the Top 10 List of Answerers? There is a difference. The TC badge/crown is the gold crown by your name/avatar which gives you the gold background on your YA profile page. You don't have that. If you had it in the past, you simply haven't met the most recent criteria for keeping it. Which is getting harder to do, since it IS based on both number of answers and Best Answers received in a specific category or subcategory.

If you truly mean the Top 10 Answerers list, that is based solely on the number of Best Answers received in that category/subcategory. You should only lose your spot there if somebody ELSE bypasses you, especially if you were near the bottom of that list. So it may not have been Los that bumped you, but somebody else that passed both of you, for example.

EDIT: OK, I checked and assume you mean in the Philosophy Top 10, where Los is. If you still have your full amount of 570 odd Best Answers and those are IN that category specifically, you should still be listed. Have any of those Best Answers been reported and removed as violations? That would be my guess. That - or the ever popular glitch. The problem is with the "endless scrolling" presentation, it is very difficult to actually count up how many BAs you have anymore.

Apparently someone racked up more best answers than you in your category.