I'm assuming we're talking NGOs here, because if we expand "organization" to include for-profit entities, then I'd argue vast majority of them will not just be US-English speaking, but
US-originating and US-headquatered.
> Without such a conscious choice, yes, Americanisms do seem a fair bit more globally pervasive and easy to fall into "by accident".
You'd need choice and enforcement - unless such organization is testing for Received Pronunciation during interviews and filtering out people who cannot into Queen's English[0], I'd wager most of the members in such org, who don't come from UK (or a few related countries), will be speaking "British English" with distinctly US pronunciation. Because while an organization can make a conscious choice here, for most people, learning a second language is a long-term endeavor that largely happens "in the background", and it's very easy to learn a blend, with UK English being present in schools, and US English everywhere else.
--
[0] - Or is it King's English now?