UK schools aren’t interested in your ECs, just your test scores, GPA and why you want to study and how well prepared you are to study the course you are applying to read.
If you’re interested in applying to multiple UK unis UCAS allows you to apply to up to 5 for £35 (IIRC). However, if you’re already using the CommonApp to apply to a bunch of US schools and just St. Andrews there’s no reason to use 2 separate application platforms.
As far as your AP and GPA questions, I’m not sure. When you say AP subject tests do you mean SAT subject tests? Or did you skip the AP classes and self study for the AP tests?
My D applied to St. Andrews without any AP scores because her school tossed APs a number of years ago. So she sent her GPA and ACT. As far as grades, they’ll be the most interested in the grades in the subject you are applying to study (read). But they’ll look at your overall academic picture because they want to know you can handle the academics. Even more than US schools, UK schools want to know you will graduate in the proper timeframe of 3 or 4 years. They’re not interested in a 6 year graduation rate.