have you not looked round RP then zenmum? if so, what did you and more importantly dd think?
they should have given you all the leavers' destinations when you were in correspondence with them about your dd's place?
tbh, it's one of the first things you should have asked them?!
they have the details on their website, it won't let me link directly but click on the school information tab and then senior schools, there's a pdf that shows what schools year 6 went to last year - www.radlett-prep.herts.sch.uk/index.html
RP is good value in that it's £7,500 p.a, so almost half the price of habs. they seem to have a good success rate according to that pdf of students going onto habs, but not for nlcs. I'd also contact them and ask for the leavers destinations' for the past 5 years as well, that will give you a good indication of how many go to habs and nlcs.
have you searched RP on MN? there's lots of info:
www.mumsnet.com/Talk/primary/a1599138-Radlett-preparatory-school-or-Manor-Lodge-School
www.mumsnet.com/Talk/primary/a1583587-Radlett-schools-and-where-to-stay
that's just two links, you can find plenty of other threads if you do a search.
from what I've heard (totally hearsay) rp is fine. don't think sports are too good and there's no hot lunches/catering facilities so dd would have to bring a packed lunch. I've heard it's very academic and not too much focus on anything else so if this matters to you great, if you want dd to have other experiences maybe not...
manor lodge is meant to be excellent and as far as I understand more selective than radlett prep - quite a few that don't get into manor lodge end up at radlett prep. again, do a search on manor lodge on mn to find out more. fees at ML are 10k so you may feel it's worth saving 2.5k a year as the leavers destinations are pretty similar. I don't think the 'overall experience' at RP is as good though and ML seems to be a better school.
there's no substitute for tours and visits though..hth