The fact that we even have year-by-year, grade-by-grade test figures for the US implies it's significantly more test-focused than the UK, where those tests simply don't exist for most grades.

Are you talking about finals or standardized tests? Because from my experience at least, the latter has minimal impact on the track that kids follow (could put on you advanced math or reading track but there is opportunity for mobility regardless) and only the SAT/ACT (highest score of however many times on chooses to take them) is used to determine where someone can go to college. But test scores (even MCAT/LSAT) will never determine what someone can study, just where, which is not the case in the UK per my understanding.

Whether you get any qualification at all in the UK is entirely determined by high-stakes standardized tests, at least on the main academic track (GCSE and A levels)