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.
https://www.bbc.com/news/articles/cn4pvyw82exoCouncil permits are usually quite public (in my country). Sneaking it in becomes part of the artwork.
(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)
The City is dead at night. If an artist wants to put art there, they'd just as somebody else said, dress up like they are workmen and be fine.
Criticizing someone of being popular is just a way to silence them. If they are popular then they are "cringe", and if they are unpopular, they can be safely ignored and that statue would have been removed by the police and forgotten without any news coverage.
Banksy may be popular, but he is not completely establishment, because well look at the statue. Its an obvious critique of the Iran war, and yet the Iran war still grinds on and UK bases continue to be used for Iran war operations. So apparently there is someone in the establishment that does not agree with Banksy. Someone boldly stepping into the void.
One of my favorite contemporary musicians is a Socialist Filipino rapper who lives in LA. I can enjoy the music while finding the ideology abhorrent because they are two separate things.
And of course there was a fucking gift shop at the end.