As a contrast to what Jack said, I am not offended when people use British English when in America. My point of view is that language is constantly evolving. Every generation sees new words come into use and old words die out. And I'm sure each generation cries foul of these changes just like the previous generation did. (Sorry not trying to start a language war here (: )

Back to the questions:

"If you will travel to GB or Australia, will be some small misunderstanding in the daily speech"

The answer depends on exactly where you go.. If I converse with someone speaking "standard" british english then we both should not have any trouble. I can get the meaning of new words and phrases from the context. On the other hand, if I spoke with someone with a very thick accent perhaps from the countryside who also uses a lot of slang, then it might be more difficult.. In fact, the area where I live has a lot of irish immigrants. If you hang out in the pubs on Friday you will hear English spoken with a very very thick irish accent, it's almost completely unintelligible to me (I can't understand it). Regarding australian english: it depends again on how thick the accent is and how much slang the person uses..

So Anatoly, tell me this - do you have trouble understanding other russian speakers from different parts of russia/former soviet states? From what i've learned so far, it seems that russian has standard pronunciation which leads to very few accents..