Parlez-vous, Anglais?

I recently download an audio book from my library for my new MP3 player. The book was an Agatha Christie Poirot mystery. In this story, the hero, Hercule Poirot, peppers his speech with French phrases. These are not your average "Mon Dieu!" Many of these expressions are central to the flow of complex dialogs and there is little context to hint at the meaning of these utterances.

I studied French for a number of years and was able to more or less follow Poirot's polyglotic outbursts. My question is - How much French is the average British reader expected to know? Does England's proximity to their oft-reviled neighbor demand a working knowledge of the tongue? Perhaps the Texan-in-the-street has a similar fluency in Spanish.