I can't get over the flag itself… It's a black flag. Not a British flag, not a white flag,… A BLACK flag.
Historically, the black flag is strongly associated with anarchism, anti-state politics, revolt, and rejection of national authority.
Had he colored it in the union jack, then I would've said it was nationalism, and the person is blinded by nationalism.
But. This is Banksy, black-and-white Banksy, so there may be no symbolism behind the black flag, but it's just very interesting. I can't accept that he would not have considered the color of the flag.
It’s Banksy. He uses color to highlight things or where the color is important. Here, I assume the flag is intentionally indistinguishable.
It's a monochrome artwork so there is no colour assigned to the flag, rather than it being specifically black.
I think it's about being slightly more subtle than a frontal attack on a specific flag.
But from an American perspective a guy wearing a suit while carrying an "anarchist" flag wouldn't be inappropriate, either.
Black flags are never depicted being wielded in this way. The stance and the clothes of the person carrying the flag are two more artistic shorthands that makes it very clear that this is a national flag, not a black flag of solidarity.
It's styled after other bronze statues that are all one colour because of the material. Given the context in which he put this up, it's a pretty clear commentary on nationalism in general, so using a specific country's flag wouldn't work.