Oh Polly you sound so worried, and I think we all are. I know of SO many off to Reading tomorrow as well - my DD2 isn't (she's already been to a couple of festivals this summer). Why is your DH expecting A all round? Is your DD the sort of pupil who is predicted these grades? My DD2 is predicted a mix of A and A results, and she will probably get them in MOST subjects. But she also knows that there are a couple or three that she may not however, those are in subjects that she will not be carrying on with at A Level and, frankly, I don't have a problem with that at all.
Certainly at the school my children are at, and the schools that their friends are at, so long as they get grade B or above in their chosen AS/A level subjects and a minimum C in maths and English, they ARE allowed to stay at their school. This is the same whether they are at private, grammar, state, wherever.
What will be will be, if she has done her true best, that should be good enough
I had 9 of DD2's friends over for 2 weeks earlier in the holidays and one of the girls in particular KNOWS that she has fucked them up, and she is also taking absolute responsibility for this. She did no work, and has come to the (very mature) conclusion, that if she as done as badly as she is expecting to, she will re take the whole of year 11 again. Even if it's just the essential ones (maths and English) she will re sit them again (and again) to get the required B/C grade.
There's nothing more that we can do. We've supported them, cajoled them, yelled at them and soothed them over the past few years of exams and coursework. The best we can do is see that what they get is what they get and if there is a subject or two (that MATTER) that could be bettered with a resit, help them through that. If it's (in our case) something like RE or ICT - fuck it. DD1 is not going to be a Vicar or a Techy! She wants to study Geography, History, English and Economics, so as long as she's hit the mark with those and her maths, anything else is good enough for me.
Sorry for rambling!