What is the average Channel 4 weekly rank for less than 2.19 million viewers on December 7, 2007?
CREATE TABLE "united_kingdom" (
  "episode_num" real,
  "title" text,
  "airdate" text,
  "viewers_millions" real,
  "channel_4_weekly_rank_a" real
);
SELECT AVG("channel_4_weekly_rank_a") FROM "united_kingdom" WHERE "viewers_millions"<2.19 AND "airdate"='december 7, 2007';