A Soviet T-34-85 tank in Southwark is rather famous and has even got its own Wikipedia entry. Originally brought to the UK from Czechoslovakia to be filmed in Richard III set in alternative 1930s Britain (hence tanks and airplanes were required), it went then to a scrap metal dealer and finally to its current owner Russell Gray, a South London developer.
The story of its settlement on the street corner is sometimes regarded as an urban myth as it's too good to be true:
It sits on a rough piece of land belonging to Russell Gray who wanted to build flats here. His planning permission was turned down and so he asked instead to build a tank. Assuming he meant septic tank, permission was granted and Mr. Gray placed a Soviet era T-34 on the land with the gun turret pointing towards the council offices.
There is no direct access to the tank as it is fenced in a kind of a community garden which doesn't look taken care of very well. Still it keeps getting a paint job from time to time and it also features some odd objects inside the fence, e.g. an old telly on the right track.
You might get an impression that it's decaying slowly (as I did myself), but then obviously a tank is a thing which is a bit hard to destroy. Take care in the new year, you steel comrade.