It would appear that something changed back in November 2013 that meant people who had some Ubisoft titles and a BT Infinity connection were unable to access www.ubi.com/uk and or uplay.ubi.com thus breaking online game play and even some single player games that relied on online access too.
Ubisoft has its own user forum with a thread on the problem and it is not clear at all who is at fault or what the problem actually is.
It seems unlikely the problem is linked to the recent problems with the Openreach FTTC modem (Huawei model), since the same modem is used with PlusNet, Sky, TalkTalk and a good number of other FTTC providers. The HomeHub appears to be ruled out also, as people with third party routers are having similar issues.
Hopefully some exposure of the problem here and other places like Eurogamer.net will get the problem fixed rather than usual thing where the blame is bounced around as the blame game is played until finally a techie gets told about the problem and fixes it. Then once fixed people can go back to enjoying the games such as Assassin's Creed, Watchdog and Might and Magic Legacy.