This premiere episode was actually screened in several UK cinemas prior to its TV transmission.
When Jack mentions that the first Torchwood HQ in London was destroyed, he is making a reference to Doomsday (2006), in which it was destroyed by the Cybermen and the Daleks.
The Hub set is composed of three physical levels. The basement serves as the autopsy area, then the main set is on the second level and Jack's office is on the top level. The rest is filled in by CGI.
The episode's working title was "Flotsam and Jetsam".
This opening episode attracted 2.4 million viewers, a good figure for a niche sci-fi show in the UK.