Yes, you apply early to your top choice and regular to the rest.
Admissions will view all your grades, although more weight will likely be applied to AS results and A2 predicted. But GCSE results are also considered. But grades are just one part of a holistic review
As far as SAT, some now require; some remain test optional; some, like Yale, offer options in lieu of SAT. I wasn’t planning on outlining which universities fall into which category since all list requirements in the admissions website and the Common App instructions