Apparently not:

  Westminster City Council has told the BBC it did not grant permission, as it was not given advance warning that Banksy's team was planning this installation.

Council permits are usually quite public (in my country). Sneaking it in becomes part of the artwork.

The trick is not to sneak it. Hi Viz and some yellow flashing lights. Couple smooth talkers.

Agreed. Also why it's totally inoffensive

(Though it's not in /the/ City of London. That wouldn't happen in a million years! City of Westminster is way more culturally flexible)

Banksy (Robin Gunningham) is the most mainstream establishment artist, while thinks himself a counter-culture revolutionary. That's what makes him so cringe. He's just another champagne socialist.