Well that’s top 0.1% right there. ACT and (P)SAT are too low ceiling to do much finer than top 1%, but as others have said, multi-level math contests will sort out the top 10^{-n} for several n.
ETA: I’m not familiar with UKMT, but a search found this Grey and Pink Kangaroo + Cayley, Hamilton and Maclaurin Olympiads (intermediate) – Mathsaurus
but I don’t see how to deduce a percentile rank.