Any advert in a public space that gives you no choice whether you see it or not is yours. It’s yours to take, re-arrange and re-use. You can do whatever you like with it. Asking for permission is like asking to keep a rock someone just threw at your head.
My conclusion from this is that we cannot limit this presumption of rights to advertising - it must apply to any message in a public space that "gives you no choice whether you see it or not". I don't ask to see Banksy's twee, ever-so-witty, pseudo-political cleverness yet by choosing graffiti as a medium he imposes it on me without me choosing to see it.
So on his logic, I shall be pasting advertising slogans onto his witty daubs at the next opportunity!
And while we're about it, remember that Banksy is a mere workman besides the genius that was "The Master of Paddington":
"Near at hand is far away in images of elsewhere"