I think one primary reason for UK-US redubs is basically language differences, and although both countries speak English, there are differences in terminoligy. For example, what we in the US would call a "Truck," people in the UK would call a "Lorry."
Case in point, Thomas the Tank Engine (though not technically animation, I know) is an example of this, especially when dealing with railroad terminoligy (trust me I'm a rail buff myself). For example, "switches" (as in track switches) in the UK are called "points," and "freight cars" are called "trucks". And that's not all that's different.
I wonder if there are terminoligy differences or other re-writes between the UK and US on OTHER British shows and movies?