American english or british english

We kind of borrowed most of it. To use one of the OP's original choices, theatre comes via modern French (théâtre) from Old French (don't know the spelling, sorry) from Latin (theatrum) from Greek (theatron). Thea is a view or action and tron is a location, so theatron a place where you can see actions happen (a theatre).

Offshoots of this were the changing meaning of "thea" to mean action as in military action. That's why we now say theatre of war. There's also the thea in anaesthesia where ana or anae means "lack of", so anaesthesia becomes the lack of a view (i.e. unconscious).

I'm English and it obviously grates that another country will tell me how to spell my own language. But it's been happening ever since written language existed and being English doesn't make me so special that I'm protected from it. Quite the opposite, in fact, because English itself isn't English. It's Greek, and Latin, and French, and Saxon, and Indian, and a whole bunch of other things.

It's just our turn to relinquish control of the lingua franca. Pun intended.

sorry about that, i didn't really notice... i never spell it like that anyway

I am English but I find myself using some American terms. Some of it is pure preference, some examples of this are; elevator instead of lift, asphalt instead of tarmac and apartment instead of flat. Then there is the influence from the Norwegians around me (I live in Norway now) as they use a *lot* of the American terms they have learned from films. Finally there is the influence films and literature from America has had on me directly. I like the sound of both but I do point out to my Norwegian friends the differences. I celebrate diversity and I like that American English differs from British English. Please do not forget Australian, New Zealand and Canadian English either. There are probably others who I have forgotten.