American Spelling. We spread our freedom to so many places that it's only logical the spelling switches to the US version. It's also more modern and probably spoken by more people than the UK English. Slowly but surely, we are taking over. Who's the world power now, Tommy? We're not going to "annexe" Canada, we're going to annex it. Goddamn redcoats.
British Spelling. This is how it normally is and this is how it should stay. Besides, the differences are so subtle that it shouldn't even matter. Typical Americans thinking the world revolves around them.
Well, we do have most of the English speaking people on the planet. If your going to use a language, using the one that the majority uses is usually a safe bet. Though they are the same language so I guess it doesn't really matter.
American English happens to include terms from a long list of other languages as well, words that wouldn't be found in a British English dictionary. It has combined the many languages spoken by U.S. citizens from many nations, even native Americans.
England is where English comes from, so it only makes since to have that English remain the standard English.
Actually, the verb is 'annex' worldwide. It's only when a small building is added to a big building that it's 'annexe'.