Not sure I agree it’s “most likely” when the linked article presents no evidence of LEA awareness or complicity, just one person speculating.
I know firsthand what can be done with a hardhat, clipboard, and high-viz vest. IMO it is far more likely that Banksy is just really good at social engineering in ways that other street artists are not.
I imagine this just isn't that difficult to get away with. Most areas are basically empty in the early hours of the morning (even in the middle of the city). And people doing some kind of engineering or installation work at that time would also not be that unusual.
The difference is that you'd get a police visit and your artwork torn down if you're not Banksy.
Plus this is pretty much the only street artist with worldwide name recognition; of course things are going to be different.
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Yes, London is famously free of surveillance and the Met is famously tolerant of political speech. Certainly, if someone had put up a statue of a pro-Palestine protester being blinded by a flag Sadiq Khan would just stand around being puzzled and letting things be. No question about it.