Royal Aeronautical Society
Address: No. 4 Hamilton Place, London
Nearest Tube: Hyde Park Corner
The Royal Aeronautical Society, also known as the RAeS, is a British-founded multidisciplinary professional institution dedicated to the global aerospace community. It is the oldest aeronautical society in the world. (Wiki)
It is a nice building with a terrace overlooking Hyde Park area. It is so-so, you can probably find better venues.