GPA isn’t a factor in UK admissions. you will need standardised scores, ideally APs, and for most of those you are looking at scores of 5 will be needed in at least three, and in some cases five, subjects. Gnerally the universities will say what they want. Sat 2s are accepted in lieu of APs by some universities, but not all. what are your current stats?
transferring between faculties is very difficult if not impossible; you really need to be sure what you want to study before applying. the SCottish universities are a bit more flexible.