Just as a quick follow up to my prior comment, both Oxford and even more so Cambridge were much more sticklers for having the necessary APs for your course, which was going to be a real pain for my S24 as his high school did not offer most of those APs (or many in general).
And then Oxbridge courses often have additional entrance exams, and then there are interviews which are no joke at all and require a lot of preparation.
Conversely, he actually applied to St Andrews through the Common App, and it basically just required a Personal Statement that was more or less just a “Why this major at this college?” essay. We would have done UCAS if he wanted to apply to more UK universities (which was a thought at one point), but it sure was easy just to apply to St Andrews as part of his normal process.
Edit: Oh, and I don’t know if this really mattered, but St Andrews was the only UK uni to send a rep to his college fair, and actually he spoke to that rep twice (as a rising junior and as a rising senior). Again, I think there was some comfort in that level of mutual interest, and also the second time around the rep was able to answer practical questions, including about APs and what that Personal Statement should be like. Definitely helped make it all easy.