I am really bemused by the way this thread has developed.
Firstly Rain, well done to you & your DD - she already has a love of reading and isn't that the best thing ever 
I disagree that most schools schemes stop at level 14, I think most stop at 11, which is lime, and for most schools after this the children are free readers, choosing from the library. Lime is supposedly NC level 3c, although that is of course debatable.
DDs school goes beyond lime, she is level 12 and having glanced at the first chapter of LS she could indeed read it. However there are a number of words that I'm fairly sure she couldn't define:
Participate
Unintelligible
Retained
Resourceful
Devices
Emerging
Absently
Previous (she might be able to define this)
That's just under 1 a page, and earlier on you mentioned your DD only needed help with a word every couple of pages. My DD is 5 so I can see it's quite possible for a child of 6 who is not bothered by scary, spooky & macarbe things to enjoy reading LS. I doubt my DD will want to read LS until about 9 or 10 as she is the total opposite of your DD. I don't know how complex the plots become so can't comment regarding that but the first chapter, which tends to give an idea , seemed straightforward enough. As a counter example I looked at the first chapter of 5 Children & It hoping it could be bedtime story fodder (me reading) but not yet, the prose is just too complex and I think DD would lose interest (whilst Charlie & the Chocolate Factory and The Wonderful Wizzard of Oz went down a storm).
Now something I really don't understand is that you say you don't want her to move up as she'll have to read longer books which will cut into her own book reading time. I don't understand because she's going to have to read these longer books sometime, if not now, then when, she's only going to gain in competence and surely the scheme books will appeal to her even less as time goes on. For your DDs sake I would go in and talk to the school to get her to free reading as soon as possible because otherwise you are only delaying the inevitable.
Equally you could take Rabbit's advice and just not read the school books!
I would, at the very least, get specific info from the school about what comprehension they want her to work on so that, whatever you decide about what she reads, you can write in her reading record how she is progressing against those comprehension goals and show that you are supporting the school too.
If she doesn't like answering your questions then you could try a comprehension workbook to support her in developing the higher level skills, especially given that the scheme books don't give much opportunity to do so until about level 8 or 9.