If your daughter passes she still might get a place. RLS offered up to 10.485 miles last year (see http://www.buckscc.gov.uk/bcc/get//asse ... ofiles.pdf
I think it all depends on the number of places available, since - after other criteria, e.g. siblings, are taken into account - unfilled places are offered to successful out-of-area candidates in order of distance from the school regardless of VR score. Presumably, this distance should change each year depending on how many successful out-of-area candidates there are and how far away from the school they live.
Best of luck to your daughter and please let us know how she gets on - we also live in Northants and have a daughter at RLS!