I went to graduate school in the physical sciences in the US and have many friends in academia in the US, mostly in the physical sciences. St. Andrews has an excellent reputation in the physical sciences community for undergraduate degrees in chemistry, physics, math and computer science. No one in those fields in the US has had anything but good things to say to us about the school. And when I look at the course work and lab work, I personally know how rigorous it is. Yes, a student applying to grad school from Cambridge or Oxford will have a edge bit the next tier down in those fields is only going to be slightly less favourably viewed given good grades and test scores.