I’ve learned after all these years that the differences between American and British language are very many and often very subtle, and thus it’s extremely difficult for an American to provide 100% convincing dialogue for a Brit, and vice versa. Klara and the Sun proves the point.
Maybe I’ve read too many books by Brits, but some of these sound to me as much American as British. I use clever and others have even used it of me. Certainly Americans sometimes give it a go. But the point is well taken. If I ever wrote a novel, I would make sure all the characters were American. I’d also be careful about regional characters.