UK university admissions (except Oxbridge and a couple of other tippy tops, and certain courses, such as medicine) are typically very straight forward for international students: if you meet the admissions requirements, have a decent Personal Statement (essay about why you are a good candidate for the specific course that you are applying to) and a decent Letter of Reference, you are likely to get an offer. So, yes it is ‘easy’ (and easier for internationals, who bring useful $$, than for locals). Note that although Manchester and Bristol are fine universities, they are more like state flagships than “Ivy League”- think UMi, UMd-CP, etc.
Most UK universities don’t ask for GPA; if they do it is usually a pretty low number- and they do it b/c US students expect it, not b/c they really care that much (except St Andrews in Scotland, who are very attuned to Americans). They won’t care about weighted or unweighted.
What they will care about is your specific standardized test scores. You need to cross-check the requirements for international students with the requirements for the specific course you are going to apply to: typically the APs/Subject tests either should or have to align with the course. So, if you are applying for a STEM course, quantitative subjects; for a humanities course, essay based exams. Very often there will be 1 or more specific subjects preferred or required.
Which brings up the course: you apply to study a specific subject, and (especially in England, vs Scotland) that is all you study. What you study, in every year of every course, is laid out in detail on university website. Although there is overlap (especially in core stem courses, obviously), there are often big differences between universities, so it is important to read the course descriptions very, very carefully before choosing what you apply to. Depending on the university and the course, it can be hard to change once you have started. So, you need to be very sure what you want to study before you apply.
Then there are the differences between university in the US and UK. In first year you will have few- if any- choices in what classes you take (they will all be directly related to your subject) and most likely 100% of your grade will be the final exam (in 2nd / 3rd year the % may be lower,). Your lectures will take up about 20% of your time- the rest of the time you are working independently. So, it requires a high degree of motivation and self-discipline. They won’t mind you. Also, while UK universities typically do have housing (especially for international students), it is not the same campus-centric experience that the US is. There is nothing like Big 10 or ‘greek life’.
Finally, there is no financial aid (though most major UK universities are accredited for the FAFSA $5K loan). You typically have to demonstrate that you have the means to pay for university in order to get the student visa. Compared to US prices, it can be good value (esp as it is usually only 3 years), but important to know up front.
From your other post you are in Grade 9, so you have a lot of time to think about this.