When was the first UK broadcast for the episode with an official TNS Gallup rating of 1575000?
create table table_26591309_2 (first_broadcast_uk___bbc_four__ varchar, official_tns_gallup_ratings varchar, PRIMARY KEY (first_broadcast_uk___bbc_four__))
select first_broadcast_uk___bbc_four__ from table_26591309_2 where official_tns_gallup_ratings = 1575000