Assassin’s Creed III is currently unavailable from Steam in the U.K. and Far Cry 3 will also be unavailable once it launches on November 30.
An Ubisoft rep confirmed to PCGameN that the publisher is currently in talks with Valve over the titles, but failed to offer a reason as to why. “We’ve been in discussions with Valve about Assassin’s Creed III and Far Cry 3, but for the time being the games are not available via Steam in the UK,” Ubisoft said. “In the meantime, U.K. customers wishing to purchase either of these games can do so by visiting the Uplay store, our retail partners or other digital distributors.”
“Assassin’s Creed III and Far Cry 3 are available on Steam in other regions outside the U.K.” So if you’re in the U.K. the only way you can purchase a copy on PC is either at retail or via Ubisoft’s own Uplay store.
If I had to guess I would say the reason for these titles being pulled is similar to what happened between Valve and EA over Battlefield 3, with the discussion being centered around DLC sales.