“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,” Ubisoft told PCGN. “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.”
EA have previously explained that a desire to retain full control of the patching and updating process motivated their split with Steam. "We take direct responsibility for providing patches, updates, additional content and other services to our players. You are connecting to our servers, and we want to establish on ongoing relationship with you" wrote Origin head, David DeMartini on the Origin blog back in July 2011.
Ubisoft have rebooted their Uplay service recently, and its verification procedures and achievements are built into FC3 and AC3. It's odd that they'd disappear from the UK store and remain available in the US. Hopefully they'll re-appear, as Brink eventually did.