How very odd. 'Smart' doesn't strike me as in any way an Americanism. I wasn't aware of any usage difference. It's just another everyday synonym, intelligent = smart = clever.
Perhaps it's not the word of first choice, I don't know; I'd say someone was intelligent, as the most neutral. I use 'clever' to mean intelligent but it also has a possible pejorative connotation of overly-clever; 'smart' is open to the same objection: you can reprimand someone 'Don't be clever!' or 'Stop being smart!'. I assume these connotations are US also. But the basic meaning is (as far as I'm aware), just 'intelligent', and is a natural word for it here.
It doesn't strike me as parallel to either of these cases: (i) Two words, both used in both countries, but each preferring one. So 'frock' and 'dress' both used and understood, but US would usually choose 'dress', UK 'frock'. (ii) One word, primary sense different in each country, but both senses understood in both. So UK 'mad' = insane, but also understood as angry*; US conversely.
Perhaps my second paragraph means it's actually of type (i) but I'm no longer as sensitive to non-UK usage as I used to be.
* This formerly said crazy, which was a typo -- I meant angry, of course.
