I once heard a quote that went something like "the US and UK are two countries separated by a common language."

I know cockney (The Beatles) and what ever Michael Caine speaks accents. Oh and Scottish and Irish accents, but there are probably many sub versions of those as well! I can understand most except when it comes to when the speaker's style are mumble-slur words together at a high rate--then again I have a hard time with Americans who do that!
Then of course most of the slang and colloquialisms go right over my head...