There is a considerable amount of graffiti on hoardings along the River Thames. The image above was of the Queen Elizabeth II Bridge which spans the Thames between Essex and Kent, taken from the Thames, and is really only visible from the river or rough ground by the river.
The graffiti shows a great deal of artistic merit and appears to not cause any harm. I don't know if the graffiti artists had permisiion or not but there is a significant amount in stretches down the river towards London forming what could be called an outdoor graffiti gallery.
Some close up shots of the 'blocks' of graffiti along the River Thames are shown below: