Welcome to UK admissions, which is so many ways much more straightforward than US admissions!
First things first: UK unis expect you to be very independent. They will make all the info you need available, but you are expected to be able to find it / figure it out. This is not a small thing: although there are a certain number of things that are done for international students & first years (aka freshers), it is very little compared to a US uni.
So, your first step is research, and here the UK unis make it really easy- the amount of info on UK uni websites is dazzling (though sometimes you have to dig a bit / a lot, and unfortunately I find KCL’s website one of the least useful). The answer to your first question (re: SAT) is different for different universities, and it is available on each university’s website.
Your second question (PS for your subject(s)) starts with the UCAS site, which has tips on PSs, but you also need to go back to the uni websites, because you have some big decisions to make: are you applying to study History? or English? or History + English? (pro tip 1- not all unis offer History + English). Your PS is an academic exercise, demonstrating why you are a good fit for the course you are applying to. Writing a PS for all three (why you are really, truly a Hx person / an Eng person / a Hx + Eng person) is not impossible, but it will take some finesse. So, go read the course descriptions really, really carefully, b/c the courses do not have much flexibility (pro tip 2: Scottish unis have a tiny bit more flexibility than English ones, b/c most courses are 4 years, vs 3 as in England), and you will want to pick ones that you are really interested in.
C’mon back after you’ve done some more homework- there are a bunch of us here who have navigated these waters & we really are happy to help