Video of the day -- accent of choice

I'm actually rather intrigued by this.  Why is the British accent the preferred one in such cases?  There's a somewhat similar situation in Arabic with the Egyptian dialect, but the reasons are pretty straightforward.  I wonder if, say, the accents of Portugal and Spain have similar prestige in Brazil and Spanish-speaking Latin America.

[Found via Crooks and Liars.  The video is a tad NSFW here and there.]