Looks like those in the UK wanting physical copies of Left 4 Dead 2 should be able to get one by Friday.
According to a post on Valve's official blog:
Due to high demand for L4D2, we decided to move various street dates forward for a coordinated worldwide release—except in the UK, where this was not possible (copies of the game are still arriving).
The good news is that we plan to have everyone in the UK playing Left 4 Dead 2 by Friday morning. Those who pre-ordered in the UK before the worldwide launch have been given access to the game.
So there ya have it.