Is it really unique to Britain? It's the first definition on Wiktionary and it doesn't mention anything about it being regional. Is scheme really only negatively in America?
It might be used this way in other commonwealth countries. In the US we'd use plan, project (personal endeavor), or program (government endeavor) instead.
Scheme itself means the same thing but it's fallen out of use and the only times I've encountered it is when the speaker wants to distance themself from it: get rich quick scheme, hare-brained scheme, nefarious scheme, malicious scheme, etc.
Yes. The Wiktionary entry mentions this: "In the US, generally has devious connotations, while in the UK, frequently used as a neutral term for projects"
Scheme itself means the same thing but it's fallen out of use and the only times I've encountered it is when the speaker wants to distance themself from it: get rich quick scheme, hare-brained scheme, nefarious scheme, malicious scheme, etc.