St As does heavy outreach to the US, and thus pays attention to GPA and ECs in the way that most UK unis don’t.
All UK unis admit by department, not university, which means that your app is evaluated by the people who teach the subject. That’s part of why @Conformist1688 said you have to pick your subject carefully.
The other part is that UK courses are much more structured and prescriptive than in the US: a course called (say) “English” will have more differences than you might imagine across different unis. You have to spend some time reading what the focus of the course is, and even what the required courses are year by year (all available online) to pick the ones you are most interested in (Scotland has a lot more wiggle room, b/c of the standard 4th year, but is still a lot more restricted than what you are used to).
It also is why the degree of difficulty of admission will vary - each course will have a specific number of places available. Some (such as IR) are wildly popular and have a higher # of applicants, which in turn raises the standard for who get an offer.
As @Conformist1688 pointed out, international $$ are very welcome at most UK unis, so a US student who meets the typical offer for the course (with a credible PS & LoRs) will generally get an offer from most UK unis. Notable exceptions include IR at StAs & Edi, and any course at Oxbridge, LSE and Imperial.
One last thing: The relevance of your APs/Subject tests (for those unis who accept them) to the subject you are applying to study is important. So, if you apply for English, Lit & Lang are important, next will be other essay-based subjects (such as the historys). The norm in the UK is to make offers based on predicted A level exam results (which students sit in June of final year). The offer is confirmed when the exam results come out in August (A levels) or July (APs). Note that each uni makes it’s own offer, and they get to pick from any of your achieved or predicted exam results. This is a long way of saying, depending on the subject and your achieved AP results you MIGHT get an ‘unconditional’ offer, but you might also get an offer ‘conditional’ on the results of one to three of your senior year APs.
Places to research (depending on your stats) include well known names- Oxford/Cambridge (you can only apply to one), LSE, UCL, Kings College London, Edinburgh and St Andrews. Others that I suggest considering seriously are Durham (which is collegiate, like Oxbridge), Exeter (strong English department, nice undergrad experience) and Glasgow (which has a tip-top English + Theatre course, and is an undiscovered gem of a college experience). In the Republic of Ireland, Trinity College Dublin also has a strong English + Theatre course. TCD is very US friendly.