Table schema:
CREATE TABLE table_26591309_2 (first_broadcast_uk___bbc_four__ VARCHAR, official_tns_gallup_ratings VARCHAR)

Question: When was the first UK broadcast for the episode with an official TNS Gallup rating of 1575000?
SQL: SELECT first_broadcast_uk___bbc_four__ FROM table_26591309_2 WHERE official_tns_gallup_ratings = 1575000