They’ll use GCSE results and predicted AS/A Level results. That will form your GPA. Everything in the past 4 years “counts”. Basically they expect lots of 6s and 7s. For moderately selective universities 4s and 5s may be fine.
Then, anything with sub-25% acceptance rates will expect AAA or higher. Some state flagships may grant you credit for A level Bs and/or Cs.
Your SAT should confirm your school results but school results matter much more. At any university with a sub 25% acceptance rate you also need to be exceptional in something out of school and show some sort of in your community/school/church/temple/mosque/youth group…