The Tate Modern
How many other art gallery would have a slide? It has the Turbine Hall, which once housed the electricity generators of the old power station, is five storeys tall with 3,400 square metres of floorspace. More information can be found at: http://en.wikipedia.org/wiki/Tate_Modern
Even if you do not go inside, wonder at the outside - with the main chimney at 99m Tall. The building was the Bankside Power Station, originally designed by Sir Giles Gilbert Scott.
The cache is squeezed in between two others along the Thames - hence the name. Along the same theme the cache is squeezed onto another nearby object, which is very convenient for admiring St Paul's Cathedral and the River Thames. There is an immense amount of muggle action in the area, so please take care!!
Finally, please ensure that the cache is secure, hidden from muggles, that both sides of the log are used and you only sign with your initials to save space!
Well down to Pja_cz for the First to find - quick (1h 5mins)