According to a local police officer, the boards are the work of a street artist called 'Mobster'. Judging by this blog, it does appear to be the case.
Despite street art often being denounced as vandalism by some law enforcement, PC Dave Loveday doesn't seem too emotional about it. Companies probably pay a significant fee for a spot on one of the signs.
On Reddit, there's been a lot of conversation around 'Buy more pointless stuff'. It was posted as 'subverstising' on the London subreddit.
'Woah, that's so deep, maaan,' says one commentator.
It's not the first time something of this nature has cropped up in the capital. Another user notes a similar sighting in Upper Clapton Road from back in September 2014. It's in the same style and places further evidence on the Mobster theory.