Most of the jokes in the film you have to be British to get. Including the cornetto joke. Which explains why people from Finland find it boring.
A cornetto is an ice cream, they've been available in this country since the 60s and pretty much every big shop/small shop/petrol station/etc sells them. The joke is that he was a big fat git who automatically answered "cornetto" to the question when half asleep first thing in the morning.
There are a great many subtle jokes like that one in the film that refer to something wholly British that no-one but a Brit would understand.
