@KaffeineKitty, there is a lot of good advice in the International Students/United Kingdom forum.
The main thing on the UK side is (as you will have discerned from reading the above) that admissions is very strongly based on standardized tests, that the courses are more prescriptive than the US so you need to read the details carefully (and although it takes some work, the UK websites are super-informative- you can get info right down to what courses she would take each year), and you apply to study a specific subject. Requirements are very specific, and for all but a handful of the top unis, if you meet or slightly exceed the stated requirements it is very likely that you will get an offer. Offers are conditional (based on tests you haven’t taken yet) or unconditional (you just have to confirm your scores with official documents). Rule of thumb for translating needed marks is that an A or A* at A-level -> 5 on AP / 700+ on SATII; B -> 4 (there is variation by uni- check the international pages). Note that the tests often have to be in specific subjects, related to the subject to be studied.h
A growing number of UK universities will let you apply through the Common App, led by St Andrews, which has the most US-centric system (and highest proportion of US students). The Common App equivalent is UCAS, which is easier than the Common App! You write one Personal Statement essay (which is academically focused, and talks about why you are a good candidate for the course, not why you are a great person), get one Rec, and pay one fee for up to 5 course applications.
When you all get to the point of having specific questions come on over to the UK forum. There are a bunch of very knowledgeable posters, who between them are familiar with most of the universities that your D is likely to be considering.