but this business of 'they have to read every book at every stage' is ridiculous (and my DCs school tend towards this too, so not just criticising other schools!). Oxford University Press, or Ginn, or any of the other publishers don't just produce this many of the goodness of their hearts. They keep producing them because across the land there are literacy coordinators silly enough to who keep buying more and more of them. So for instance, if OUP publish 6 more level 9 books next week and MumNWLondon's school buy them, does this mean her DD then has to read another half dozen?
I understand the need for breadth of reading experience, but if you want quantity rather than challenge, pace & progression, you could give them Mills & Boon on that basis.