HH's 2016 results are on their website in full including both offers and places accepted http://www.hornsbyhouse.org.uk/destination-schools
For 51 leavers - 14 Alleyns offers, 13 JAGS offers, 6 Wimbledon High. To have almost half of your girls getting offers to JAGS is pretty good going especially when they aren't selective on entry.
The boys school results don't look as strong with only 5 DC offers and none for KCJS, Westminster, St Pauls...
But HH's ethos tends to attract families who are aiming for relatively local day schools anyway so that may skew where they choose to apply.
As for the large number to Whitgift, I suspect this in part reflects the fact that its exam results have been closing in on DC's a lot in recent years and its facilities and grounds are pretty spectacular. It's not an automatic 2nd choice any more. The same thing seems to be starting to happen with SCH too.