Not in London, so can't advise about that, but where we live almost every 6th form insists on a C if you want to continue to A2.
My ds did badly in Yr12, felt he'd chosen the wrong subjects, and got poor results at AS. The school was very supportive, and he redid Yr 12 with different subjects. He did well this summer, so it definitely worked for him.
However, my understanding is that the funding has changed and schools now get much less funding for a student who is in 6th form for 3 years. They used to get full funding for up to 3 years of sixth form/ FE college, but now get only partial funding for the third year. I might be wrong, so check it out, but if it is the case then schools might be reluctant to let students redo year 12.
FWIW, my ds did stick with one subject although changed the others. He got a D in it last year, but an A this year, so it can be done. However, he really did no work whatsoever in Yr12 first time round ( missed loads of lessons, didn't do homework, didn't revise) so the D was what he deserved, and everyone knew he could do much better. Is this the case for your dd or did she work really hard? if it's the latter, would redoing the year help at all? It may be that she's not really cut out for that subject..