This is a really clever idea. Clean Advertising!
Basically the artwork Dark Iris created was turned into a stencil which is then jet-washed onto the grimy streets of London. These were located all around Hampstead and Belsize Park.
You can find the company STREET ADVERTISING SERVICES here. They also do chalk pavement art too!
(Please note, I wrongly credited a different company on the original post. This has now been amended. Sorry guys!)