In my previous wander around St James I spotted a metal clad building. I've taken some photos and found a few details.
66 St James's Street was designed by Rodney Gordon of Tripos Architects in 1979-82. The building was commissioned for Swiss Bank.
The distinctive exterior is not copper as I origionally thought but bronze-anodised aluminium. The current occupants are the Stern Pissarro Gallery who moved there in 2009.
Another photo of the 66 St James Street