I don't quite agree. Many preps do prepare a lot with a lot of tests, interview prep, and teach VR and NVR, and those that don't, including ours, teach at least 1-2 ahead of state schools and often have a lot more homework.
I don't know about girls' schools, but for the very top boys' schools like St Paul, Westminster, King's and very top grammars, many parents have been preparing for years. Of course, the level of preparation differs but most are a lot more than minimal, certainly more than a few hours a week only in the last few months (the typical myth here in MN). Sure, if you go down the ranking, it will be less but that is only relative. Being new to this country for about a year, I am absolutely stunt by the competition for these top schools, and the extent some parents go into.