British English or American English?

Or Australian English, for that matter. (Canadian English is American English.)

Generally, write in your language. Ultimately, it's up to the publisher. For an international audience the standard is American English for obvious reasons. In international business documents, software documentation, and Internet publication, American English is usually preferred.

But this standard isn't closely adhered to, because all forms of English grow more alike than different today.

If somebody is going to think you erred because you write the American different than instead of the British different from, tell him or her to ditch the arrogance. American English is as English as English English. Americans have their own language and needn't conform to British standards.

Besides, stuff like that is no problem. If your publisher wants the language to conform to a particular standard, copyediting can find and fix those little things quickly and easily.

Figures of speech, or even certain words, can be a problem though! Strange misunderstandings can occur if you aren't careful with them. So, for example, try to avoid using words and expressions that mean or connote something different to Americans than to Brits.