According to this GI story, GTA IV has had the biggest UK launch day on record, as if that's any surprise. From the site:
GTA IV first day sales were 609,000 units - eclipsing the record of 501,000 units set by GTA: San Andreas in 2004.
GTA IV also set a record for first day sales Xbox 360 software, with 335,000 units. The prior record, set last September by Halo 3, was 266,000 units.
GTA IV easily eclipsed the first day PS3 sales record previously held by Gran Turismo 5: Prologue, which had day one sales of 80,000 last month. The PS3 version of GTA IV sold 274,000 units in the UK on day one.