In my experience a lot of these aren't really happening. I've never heard anyone use the word Chav, and doubt anyone would have a clue what that's about it I did try to use it. Cheeky? Nah. I think if someone called their apartment a "flat" without being British I'd laugh in their face. C'mon man.
I do say "innit" a lot... but with a fake Ali G style accent.
Actually I've found that flat IS gaining more use. I've also hear "flatmate" more often. The one caveat to all of this is that you hear these terms being used by more educated Americans that travel internationally or are likely to consume British media. That, sadly, is not most Americans. I work for an international organization, so its hardly shocking that I hear more britishisms since they are accurately described as part of "International English." For instance, holiday is more widely used outside of North America instead of vacation. The same is true for flat.
Well terms like flatmate(Housemate) means you share a property together. Whilst roommate tends to indicate you share a single room.