There is a lot of information online about US students going to the UK, including here on CC- do some searches.
Brief summary of some of the headline pieces:
*Pretty much all the info you need about each university is on their website.
*In general, UK unis are less interested in ECs, GPAs and classes not related to your intended subject. Notable exception is St Andrews.
*In general, a US student who meets the stated requirements (typically 3 APs with specific marks in relevant subjects) and does a decent Personal Statement (essay about why you want to study the specific course you are applying to) and a decent LoR is likely to get an offer. Notable exceptions are Oxbridge, LSE and Imperial.
*In general, if a course requires “AAB” at A level you can interpret that as 5,5,4 on APs . If Maths and/or further maths are specified it usually means CalcBC. Unless they specify ‘no particular subjects’, you can assume that they are expecting that your exams will be in areas close to the subject area. So, if you are applying for Chemistry, they will likely require Chem, BC, and ‘prefer’ Bio, Physics, etc.
*The courses are much more focused, much more structured and much less flexible than in the US. In most courses you have few, if any, options in your first year; when you do have options they will be almost entirely in your subject (so, if you enroll in a Math course, you are unlikely to take any non-math classes at all- ever). Note that the Scottish universities have some electives and (especially in the first year) are more flexible than the English ones. Also, a number of universities (incl Cambridge) have a broad course in natural sciences, where you start wide and narrow. It is always important to read the detail of the course at each uni to see what is required/ where the focus is: a “BA English” can be surprisingly different from one university to the next.
*Scottish and most English STEM & foreign language courses are 4 years; English (non-language) humanities tend to be three years.
*Assume that there is no meaningful financial aid.
*IMO none of those schools are so obscure that name recognition or credibility will be an issue
So, does any of the above give you any better sense of whether the UK is a good choice for you? If you want to share your interests & stats, there are a number of very UK-knowledgable posters around.