I'm pretty sure the US super elite schools (HYPS) do not cover as much of the population as the UK super elite schools (Oxbridge). I believe it's something like 1-2% for the UK and 0.4% for the US. I may misremember the components (i.e. it may be for merely elite schools, like the Russell Group or the rest of the Ivy League plus MIT and Cal Tech etc.)
Super elite term taken from Lauren Rivera's "Ivies, Extracurriculars, and Exclusion"