Why is the voice of the ugly stepsister in "Shrek 2" different in the US and UK versions?

It is simply a matter of having culturally significant voiceovers for different countries. Audiences relate better to figures they are more familiar with.

US: Larry King
UK: Jonathan Ross

You often get these types of audience-specific changes.

They messed up the credits for the UK version too. In the UK version the voice of the Ugly Stepsister is that of UK TV/Radio Presenter Jonathan Ross yet the vocal talents are mis-credited as the U.S. edition's CNN talk show host Larry King.

Also, the voice of the Red Carpet Reporter is none other than UK Pop Idol Extra presenter Kate Thornton, yet again, the voice is credited as Hollywood reporter Joan Rivers, whom the physical appearance of the character is based on.

http://www.moviemistakes.com/film4135