It’s not what he does, it’s where he does it that makes Banksy modern Britain’s most effective artist. The pseudonymous street artist from the West Country picked the perfect platform yet again this weekend – the Pyramid stage at Glastonbury festival, no less, where Stormzy headlined in a stab-proof vest decorated with a near-monochrome union jack flag. It added a punch of visual tension to his verbal mash-up of modern Britain from knife crime to Boris Johnson. Only after the rousing show did a photo of Stormzy in his discomforting vest appear on Banksy’s Instagram with a claim of responsibility: “I made a customised stab-proof vest and thought – who could possibly wear this?”, adding on a separate line: “Stormzy at Glastonbury”.

Stormzy, it turned out, had not been simply wearing a stage costume, but art.

Of course it is. How did we not see it straight away? The union jack flag has been used as a pop-art icon by musicians from The Who to the Spice Girls. Nor is the idea of body armour as art unheard of: Jake and Dinos Chapman recently exhibited a series of bronze suicide vests that imitate a bronze aqualung created in the 1980s by Jeff Koons. But these artists never thought of getting a grime star to wear their art on the main stage at Glastonbury. Stranger still, Stormzy seems to be the delighted victim of a Banksy prank judging from his tweeted response: “Absolutely fucking speechless.”

If so, how did Banksy get Stormzy to wear one of his works unknowingly? Did he leave the vest in the Glastonbury dressing room with a note: “From an anonymous well-wisher?” Surely the rapper was in on the stunt, for the vest was perfectly coordinated with the rest of his show. Its stark design showed up powerfully against the spectacular swirl of multicoloured lights and flashed-up messages. It seems too much to believe that Banksy happened to infiltrate a work of art that balanced the optics of the performance so precisely.

It’s more likely that the two are collaborators. After all, they are both masters of modern culture. Stormzy’s set began with a video discussion of the power of culture, and embraced ballet in an interlude celebrating the fact that dancers’ shoes are finally being made to suit black skin tones. Stormzy and Banksy both know that culture is positional and situational. It’s context that makes the comment. Stormzy’s show was a triumph because he so enthusiastically embraced this opportunity – enjoying “the greatest night of my life” – instead of alluding to, what looked like on TV, the overwhelming whiteness of the Glastonbury audience. Because this is about communication and Banksy’s design helped Stormzy speak for England.

Stormzy was wearing armour, alluding to contemporary Britain’s wave of knife crime. It resonated with his references to this tragedy – at one point the words “knife crime” were lit up behind him and there was an excerpt from a speech by David Lammy. The black and white tones of the flag, with just a hint of rusty red, like dried blood, were mournful. But images are never as simple as that. Banksy’s artistic weakness in the past – and it’s still a huge limitation if you look at his works in a gallery or isolate them from their context – has been a narrowness and superficiality in saying just one thing. The vest he made for Stormzy is different, though. Its combination of the British flag with an artefact of paranoia is a perfect image of our moment. Stormzy’s tense and provocative stage-garb exploited the flag’s visual strength in a new way. Amid all the stage razzmatazz, he wore the banner of a divided and frightened nation.