Ubisoft are having a fairly large assault on Christmas this year. They're due to release three big titles around the holiday period, but the listings are strangely absent from the November and December listings for Steam's UK store.
Assassin's Creed Unity will release in Europe on November 13th, Far Cry 4 on the 20th and The Crew will appear on December 2nd. This is when it will be available to purchase in stores as a hard copy and also as digital copies across the world from all digital distributors except Steam in the UK. Ubisoft say that they've been in talks with Valve but as of right now the games would not come to Steam in the UK.
The reasons behind this strange exception aren't clear, but it's likely to have something to do with the pricing. The physical store copies will be retailing for around £55 and the digital copies are only a fiver cheaper in Ubisoft's uPlay store and other digital retailers with the games selling for £49.99. Currently Steam doesn't really sell games at a price above £40 so it's likely that the high price of all the games is the sticking point.
Maybe the games will come to Steam at a later date, but if a compromise isn't reached soon Ubisoft will potentially be missing out on a little market share this Christmas from the Steam gamers.