Actually you're right, the head of examinations at the school I just took the exam at sent me these thresholds when I asked about marks and grades but I just realised it was for June 2018. However, there doesn't seem too be much variation in most sittings.
That being said, for June 2018 exams if you got below 50 marks for the extended theory (42) it wouldn't have mattered how well you did on the other two exams (even if it took you above the overall combined total required for an A) you wouldn't be able to get an A... It is called "minimum raw mark required for grade".
Hopefully because of how difficult this exam was that number is a lot lower.