You can still begin the process for the selection review, which means the HT will be asked for their view on your DD's suitability for grammar and the score they gave before the exam.
You will be sent this. You can then decide if you want to proceed or not. It is started via the link on BC website, and they send you the head's form. To proceed you have to then upload your own evidence along with the head's report. You can leave it at this stage and the review will not happen, but you at least get to see what the head says and their score.
I'm not sure where in Bucks you are. There are some better secondaries in some places than others.
If you do go for a review, you will need to include previous years reports, which show that your DD consistently got EXS in reading, writing and maths. As the school your DD is at won't have them, it will be incumbent on you to provide them.
It's also important to remember that only academic ability should be included in a review. Anything else will be discounted and may prejudice your review as they only have minutes and need evidence of academic ability, not leadership, musical ability, sporting ability, etc.
Are there any extenuating circumstances? Anything major happened or illness, that could explain why she didn't pass?
If you do have any extenuating circumstances they can be included, but bear in mind they should have been reported to the school before the exam.