Have found most of the professionals I have dealt with to be as polite as the Brits.
Take that down a level and that's where I have seen the brash shoot first ask later ignorance of the world.
But given that we brits don't always put our best foot forward its a little hard to fully quantify.
Only thing that really bugs me is the polytricks down there which seem to be bent on corruption, grift and polarisation regardless of party.
As for SLANG that has been my GOTO for years here in Canada (pinnacle of politeness) and more so when dealing with Americans.
Always makes for decent ice breakers in conversations. More so when you remind them we have sh oles older than thier "democracy"