Tomorrow, and because of GAME (and other retail stores). This is nothing new; digital distribution services have always been hindered by retail stores (which adhere to Friday release dates here) so that they don't get an unfair advantage.
What's new is that GAME essentially strong armed THQ into pulling the ability to pre-order from Steam, but only in the UK. I'm guessing that they said they straight up wouldn't stock the game on any platform unless THQ did what they said. Which is why you needed to enter a code or a URL to get the Space Marine demo on Steam in the UK: the page for the game is no longer there.