Differences in the American Version
I just got back from an advanced screening, and having seen the British version of the film as well, there were a few differences:
-A few voices are dubbed over, most notably with Anton Yelchin voicing Albino Pirate now, rather than Russell Tovey. Al Roker is the American voice for The Pirate Who Likes Kittens and Sunsets (which I believe he's called something differently in the UK version) and one of the scientists is given an American accent.
-Some jokes were changed. The bit about Darwin lamenting that he never reached second base with a lady is changed to 'never even kissed a girl'. The part with the hot air balloon is changed to 'It's great for attracting ladies', as opposed to 'great for looking down ladies' tops'. Those are the two I remember, but there were a few more "naughty" jokes which were made more family-friendly for the American version.
There are other differences, those are just the ones I remember. Just a tad bit disappointing, especially when it comes to voices being changed, but overall it's still a very funny film. If you can see it in its original British dub, though, go for it.