You should be able to appeal this under the condition that you don't believe you've been treated fairly by you institution. They'll investigate it and look at things like your mock grades. If they are A's and B's as you say, i wouldn't imagine that it would be too hard to get a fair grade in the end.
When I got my centre assessed grades, the back read
GUIDELINES FOR AN APPEAL AGAINST A CENTRE ASSESSED GRADE
1. A student can only appeal this grade/rank order on the grounds of discrimination/bias by the school. In the first instance this should be done in writing to the Principal of the school by Thursday 3rd September 2020. The appeal can only be made against the process and not the mark submitted.
2. The Principal will arrange a review to be conducted; the outcome of which will result in either:
a. no irregularity in the procedure of awarding Centre Assessed Grades and rank orders. In this case the student will be informed. The students will have an opportunity to request a review of the school’s decision not to inform the exam board.
b. irregularity in the procedure of awarding Centre Assessed Grades and rank orders. In this case the exam board will be informed immediately. The exam board could then take a decision to investigate the evidence as malpractice by the school.
3. This outcome will be communicated to the student within 14 days.
4. In exceptional circumstances, the student can appeal to the exam board directly if there is evidence of bias or discrimination.
These were the guidelines on results day, but I don't think that they're outdated. Definitely worth a try.
Hope that helped.