Agreeing (again…) with Alfonsia: St Andrews is a bit of an outlier. StAs does very well in the rankings, but the majority of the Brits I know are underwhelmed by it (and figure that the American fascination with it comes from it’s most famous alum; they don’t realize the full court press StAs has made to woo US students). The American students I know there tend to be very happy.
It can be easier for US students to get into some UK universities than it is for UK students, though not at the tippy top (Oxbridge, LSE). If you have the test scores, and don’t make a total mess of your application you will most likely get an offer. That is true for both UK & US students, but the US students use APs and SATIIs, which is a considerably more flexible process than the A level system.
Also, it can be easier for some US students to get into the tippy-top of UK universities than for that same student to get into HYPS, simply b/c the UK system allows for asymmetry and HYPS typically do not (unless you have some other off-the-charts aspect to your portfolio).