The Mappin Terraces at London Zoo (Now re-named Bear Mountain). It contains Sloth Bears and a troop of Hanuman Langurs.
Edit: the bears have now left and will be replaced by australian fauna.
Anteater Area beside what was once known as the web of life building, but seems to have changed its name to B.U.G.S. Very nice architecture, really interesting building with lots of 'green' features.