Thank you all for the really helpful comments. I am increasingly thinking that I have made a mistake to steer DD towards chemistry instead of maths, and that she should go with her first instinct which was to take maths at A level.
To answer a few questions that other posters have asked:
Yes, we can afford to pay a private tutor for extra help if that turns out to be necessary. We did that for DS for A level maths. (Even with the extra tuition, DS still found maths the hardest A level subject.)
Re the possibility of taking A level English literature, that was actually DD's lowest grade at GCSE (though still a grade 7, so not a bad result!) DD will definitely not want to take that at A level.
Re humanities subjects, the only humanities subject which DD took to GCSE was RE (which was compulsory). DD would definitely prefer to take maths (or chemistry) over RE.
I have not heard any complaints about the quality of maths teaching at the school. Although DS found maths at A level hard, he did not suggest this was due to poor teaching. I don't know the breakdown of A level maths grades for DS's year overall, but I do know that at least one student got A* for both maths and further maths.
For chemistry, there is one teacher whose teaching leaves a lot to be desired. The other chemistry teachers are good. I suspect DD's results-conscious school would avoid letting the less good teacher loose on an A level class, but I guess we can't rule it out, so it is a point to bear in mind.