Idiosyncratic differences
Figures of speech
Both BrE and AmE use the expression "I couldn't care less" to mean the speaker does not care at all.
In both areas, saying, "I don't mind" often means, "I'm not annoyed" (for example, by someone's smoking), while "I don't care" often means, "The matter is trivial or boring". However, in answering a question such as "Tea or coffee?", if either alternative is equally acceptable an American may answer, "I don't care" while a British person may answer, "I don't mind". Either sounds odd to the other.