Cotman in Normandy, Dulwich Picture Gallery, London

Laurence Binyon may be best known for penning the Remembrance Day poem, "They shall grow not old, as we that are left grow old", but he was also keeper of prints and drawings at the British Museum. In 1933, in his book English Water Colours, he made a startling claim. Modernism, or at least the British kind, was not a beastly French habit imported by Roger Fry in 1910: it was home-grown. "There was no need [in Britain] to invoke Cézanne," Binyon boomed, "for Cotman was there to show the way."

Read 80 years later, these words seem preposterous. John Sell Cotman (1782-1842) may have been many things – a watercolourist, a fine draughtsman, unfairly overlooked – but a rival to Cézanne? No. Why would Binyon, a clever and cosmopolitan man, say such a thing?

It's something to think about as you walk around the Dulwich Picture Gallery's show Cotman in Normandy. If you know Cotman's work, then it is likely to be his Romantic landscapes, pictures such as The Waterfall, all Byronesque Sturm und Drang, or views of Brecknock (now Brecon), alive with a Romantic taste for the picturesque.

Around the time he was painting the first of these, Cotman's style went through a radical, and still unexplained, change. Instead of cranking up the contrast button until every scene became a battle of dark and light, he started seeing life through a lemony haze. Had Cotman followed his contemporaries to Italy, this sunniness might have been understandable. But he didn't. His life was spent between London and his native Norfolk, and he seldom strayed further than Wales.

Except, that is, to Normandy, which he visited three times between 1817 and 1820. The output of these trips forms the core of the DPG's show. In 1822, Cotman published two volumes of etchings called Architectural Antiquities of Normandy: a few are on the gallery's walls, although most are preparatory drawings. Cotman's departure from landscape wasn't new – he had had a Romantic's yen for English ruins, particularly East Anglian ones – and many of his landscapes included buildings. But crossing the Channel to see what lay on the other side was certainly a novelty, given that the French had been Napoleonic bogeymen until 1815.

If you're not getting Cotman as a proto-Modernist yet, then hang in there. Although the DPG's curators hint that his willingness to see France as more than a bugaboo was itself modern, what the Norwich boy found was an even older stereotype of French beastliness. As in Norfolk, Cotman's eye was drawn to ruins rather than to architecture per se, and the ruination of Normandy's churches had been visited on them by the Revolution. If Cotman saw past the horrors of Boney and his men, it was to focus on those of the Terror. In a political sense, the works of Cotman in Normandy are deeply conservative. They have little to do with the modern life prescribed by Baudelaire 40 years later as the proper subject for an up-to-date painter.

So what was Binyon on about? This question matters, because it was he who led the reappraisal of Cotman in the 1930s, and it is his view of him that has stuck. The temptation is to dismiss the poet as a bit of an old buffer, his views coloured by the Little-England blimpishness of his day. As the situation in Europe grew darker in the 1930s, so many critics pulled up their aesthetic drawbridges. Cézanne? Man's a damn frog. Cotman is the Modernist for me.

But, as I say, Binyon wasn't like that. For an idea of what he meant about Cotman, we have to divide content from form. However historically backward-looking the works in this show, Cotman's treatment of them is hugely experimental. It's not so much his yellow light as his entirely novel way of putting together a composition. You can see it here in works as unalike as Crypt in the Church of St Gervais, Rouen and Domfront, from the Tertre Grisière.

What fascinates Cotman is the architecture of the space he's representing, whether it is natural or man-made. And this fascination spills over into the way he constructs the space of his pictures – not as a gradually diminishing thing, its recession marked by a progressive change of colour, but as a series of discrete planes, each quite distinct from the one beyond it. That flattening-out is certainly modern, although it is coincidentally so: Cézanne may have had similar interests, but for very different reasons. And while Cézanne's planar tastes led to Cubism, Cotman's went nowhere very much. I doubt you'll find many signs of them in the work of his best-known pupil, Dante Gabriel Rossetti. Still, this is a fascinating show, and a good introduction to Cotman if you don't already know him.

To 13 Jan (020-8693 5254)

London’s Lazarides Gallery teams up with the Old Vic Tunnels to present Bedlam, an installation exploring the fine line between genius and madness (till 21 Oct). David Zwirner has just opened a new gallery in the capital, and the first show – Allo! – is by modern master Luc Tuymans (till 17 Nov).