Yes, you're right, it's a tank and it's parked close to the centre of London. Even worse, it's a Russian T34. And don't ask me what the guy was on who painted in zebra stripes! There is a back-story to this and I am grateful to Michael Jenkins at Samizdata for telling it. Apparently, a local property developer bought the land with the aim of building a house or flats on the site but was refused planning permission by the local council. He bought the tank cheaply from the company that made the film of Richard III starring Ian McKellan which, if you remember, was set in a proto-fascist Britain of the 1930s. You can watch the trailer here and even catch a glimpse of the starring tank itself. The film was superb, the very best adaptation of Shakespeare's play to modern times I have ever seen.
As Mr. Jennings notes, sardonically, "As I said, London contains some weird shit. I can’t imagine living anywhere else."