Super smart but lazy comes with pros and cons when comparing the US and UK. Someone who is able to memorize everything for end of term/year exams but doesn’t bother to turn in homework is likely to do better in the UK. But someone who doesn’t know how to study is likely to do much worse: UK students are expected to spend multiple weeks revising on their own for high stakes exams even in high school. And you may not find out anything is wrong until you see the end of year exam results.
I would also be a little wary of St Andrews as the Scottish kids are super smart and motivated (it is far more competitive for them as they don’t pay fees if they stay in Scotland) while admissions for Americans who pay a lot are relatively easy, so the foreigners may not be as talented. Our neighbor’s kid (who was also smart but has ADHD) found it very challenging to keep up there.
I’d also think that working in the US after college is likely to be more attractive than the UK as salaries are much higher: my S is in real estate consulting and gets paid well (albeit he’s working 50 hours a week pretty regularly). He applied to UK colleges (mainly interested in Oxford - we are British originally) but he’s glad he stayed in the US as he was able to focus much more on his interests (housing policy). I’d note he got almost no handholding at his public flagship (UCLA).