Usually the UK schools that send a meaningful portion to US ivy schools are the top schools in (West/Central) London such as St Pauls, Westminster, and to a lesser extent KCS, or boarding schools having a large proportion of overseas students such as Brighton and Cheltenham. That is, such schools are both academically strong and have an international-minded (and deep pocketed) demographics.
Why are so many Sevenoaks students going to Ivy? I understand it is a good school in a wealthy area, but does it have a large international population? My impression is it is similar to Brighton and CLC, but its Ivy leaver number is very high and even close to that of St Pauls.
www.sevenoaksschool.org/academic/beyond-sevenoaks/university-destinations/