It appears that some sort of conflict between Ubisoft and Valve is keeping Far Cry 3 and Assassin's Creed III off the UK version of Steam (while the games are available in other countries on Steam), PC Games N reports.
They followed up with questions directed at both Ubisoft and Steam, with Ubisoft making the following statement:
"We've been in discussions with Valve about Assassin's Creed 3 and Far Cry 3, but for the time being the games are not available via Steam in the UK."
"In the meantime, UK 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 3 and Far Cry 3 are available on Steam in other regions outside the UK."
Meanwhile, Steam took the approach that partners can do as they please. Remains to be seen if this is a sign that Ubisoft may walk the way of EA and opt out of Steam in the future, even if it appears more likely in this case that there is some local issue to resolve as the games are available on Steam elsewhere in the world.