St James Theatre is the first newly-built theatre complex in central London in 30 years, built on the site of the former Westminster Palace. Though from the outside it looks a bit like a mid-priced hotel, the 312-seat main house has been an interesting addition to the London theatre scene. Though it's produced plenty of original productions, it's perhaps most valuably served as a place for acclaimed regional productions to swiftly transfer into.
It also has a 150-capacity studio that's carved out a decent niche for itself hosting musical cabaret and suchlike.