Cameron is shown as a diner at the Chez Angela restaurant, which can be seen as representing the European Union. The owner/manager (German Chancellor Angela Merkel) is taking Cameron's order. Cameron tells her, "Roast beef with Yorkshire pudding, followed by spotted dick ... or I'm leaving". These are, of course, traditional English dishes — not what you would expect to find in a European restaurant. The other diners (i.e., other EU members) react with horror at Cameron's 'faux pas', while the chef (French president François Hollande) looks on from the kitchen.
To explain the cartoon, we could use another food-based metaphor. Cameron wants to have his cake and eat it, i.e., he wants Britain to remain in the EU, but only if he can negotiate a new deal with Brussels which would give the UK better membership terms.
The cartoon is inspired by the work of H.M. Bateman, the 20th century British humorous artist and cartoonist noted for his "The Man Who..." series of cartoons, featuring comically exaggerated reactions to minor and usually upper-class social gaffes, such as "The Man Who Lit His Cigar Before the Loyal Toast", "The Man Who Threw a Snowball at St. Moritz" and "The Boy Who Breathed on the Glass at the British Museum." (source: WIkipedia)
• Your guide to the European referendum (Chris Riddell cartoon from The Observer)