Oxo Tower Wharf
Bargehouse Street
London, SE1
Tel: 0207 401 3610
Website: http://www.oxotower.co.uk 

You can get the most amazing views of the River Thames and the London skyline from this gallery in the OXO Tower.  It is not widely advertised,  and to get to the entrance it looks as though you are going to walk through the adjacent restaurant.  Just keep going to the front of the building and out on the gallery  - be prepared to be stunned by the panorama of London in front of you. 
Nearest Tube station: Blackfriars or Southwark.

The Oxo Tower on the right of the picture, which shows the Thames from Gabriel's Wharf on the South Bank. Photo by Colin Smith and reproduced by kind permission