You are quite right. The basics of English culture are probably all the same, but if you would go to Australia, for example, you would find words and expressions being used quite differently from those in Canada.
Just to give you an idea - do you know what a 'doona' is? Well, in Australia, it's a duvet and a 'dunnie' is a toilet. The bottom line is, that your students must be made aware of the fact that colloquial expressions and slang in each English speaking country can be very different, and that even the accents can pose a problem in understanding.
I am a native English speaker and I have lived in a few English- speaking countries in my life, and I can tell you that sometimes I barely understood a word of telephone conversation in Australia!!
