In summer 1795, George Canning, future prime minister, records some tantalising news in his diary. "Mr Gillray has been much solicited to publish a caricature of me and intends doing so!" he swanks. A month passes without the promised glory. He admits to concern. By Christmas, there is still no image. In the end, Canning waits an agonising year – furtively checking the print shops – for the negligible honour of appearing as a corpse in Gillray's Promised Horrors of the French Invasion. But by then vanity has devoured him. You might say he has already turned into a Gillray caricature. Life imitating art: that is one measure of a caricaturist's genius. Another is the masochism of his victims – the celebrity buying her own Spitting Image puppet, the politician hoping to appear in a Steve Bell cartoon. In which respect, as in so many others, Gillray still reigns supreme, this artist who could mock the Queen as a syphilitic crone with wizened dugs bared, talons clawing at the prime minister's crotch, and still find eager patrons at the palace. You will not see this devastating image in Rude Britannia at Tate Britain. Perhaps the curators did not find it comic. Their Gillray is the schoolbook satirist, the man who drew Pitt and Napoleon carving up the plum pudding of the world and that dangerous revolutionary Charles James Fox with no trousers (sans-culottes? Remember?) Likewise, their Hogarth is the author of those history lesson classics, Gin Lane and Marriage