There is no generally accepted rules of politeness when it comes to british or American english. It can only be there in the demeanor of the people. When there was no America as it is today, there was only British English, and at that time, English was spoken with a lot of grace, i.e, very formal.
However with the advent of American English (Not necessarily causal, just corelative) informal expressions came to the fore and began to be used more and more in everyday English. This may have lead some people to believe that American English is less polite, but that is an assumption at best. You can be polite or impolite in any language. It has got nothing to do with the language.
Now, if you want to know more examples of politeness in daily lives (or the lack of it) by English people, read these links :)