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.
Pretty much what we learned as student when we were doing something which we technically had no permit for (like digging out some stuff, using it for a theme party and putting it backs few days later). Put on some hiviz and nobody is the wiser.
I live in Westminster and we are officially supposed to put our rubbish on the pavement and there are usually no police around. They are just lucky it wasn't taken to recycling in the morning.
It doesn't make sense in the City. Waterloo Place, where he put this, has a bunch of statues already for tourists to gawp at, just now as well as "Bloke on a Horse who was an important military leader" there's this guy stepping off his plinth because the flag blocks him from seeing what's in front of him.
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.
Council permits are usually quite public (in my country). Sneaking it in becomes part of the artwork.