The sort of English you’ll see in literature, newspapers, any remotely formal communication, in grammars (which learning materials are based on as well). The stuff learners will aim to learn.

Differences between US and UK English, and the dialectal variety within each of them, is not all that relevant here. Where I live, students are taught British English, but no professor ever chastised us for using American pronunciation or vocabulary. Both are within the range of what natives will find acceptable.