In Ameilia Rauser’s paper  I found an interesting exposé about caricatural prints in the late 18th century: “This print is part of a longer tradition of gastronomic contrasts, in which fat Britons were identified with roast beef and porter, and skinny Frenchmen with frogs, garlic, snails, and leeks”.
The engraving she is referring to was said to be in the British Library, but is actually in the British Museum, Dept. Prints and Drawings  and was made by James Gillray in 1792. It is named ‘French Liberty, British Slavery’. The snails are very stylised, both dextral and sinistral.
And this is the original print:
For a more fully, contextual explanation, please see Rauser’s paper (p. 100–101).
 Rauser, A. (2005). The Britishness of caricature in revolutionary France. In: Ogée, F. (ed.) ‘Better in France?’: The circulation of ideas across the Channel in : 88–110. Bucknell University Press, Lewisburg.
 BM, J3.14 (http://bit.ly/1eNMnxo). I’m grateful to Adrian Shindler of British Library, Humanities Reference Services for pointing me the right way.