Rowlandson’s 1790 catalog of everyday life among the Brits includes a picnic scene. People looking at it would recognize an alfresco luncheon, but they would not have a name for it. What we call a picnic was unknown in English. Even if Brits knew the French word pique-nique, it would denote indoor dining at home, a salon, or a restaurant.
By including the alfresco scene in Picturesque Studies and Scenes of Everyday Life and People, Rowlandson acknowledges this gathering among the gentry rather than ordinary folks.
There is a tent for shade and a cloth with food and bottles of wine beside it. The party of nine includes four couples and a single woman. All have come by boat that is moored on the shore. It is a genteel affair that is neither rustic nor ornately artificial. There would be servants, but they are not visible. The attire is informal, though two men are wearing military uniforms.
See Catalogue of Books Illustrated by Thomas Rowlandson. New York: The Grolier Club 1916; https://archive.org/details/ofbookscatrowlan00grolrich; Thomas Rowlandson. Plate 7: Picturesque Studies and Scenes of Everyday Life and People (1790). London: S.W. Fores. Royal Collection Trust/© Her Majesty Queen Elizabeth II 2014