If I was using an educational consultant I would use the Gabbitas trust or the Good Schools guide.
How old are your children, and when are their birthdays? If your DD was 11 before 1st September this year she would be in year 7. If she turned 11 after that she would be in year 6.
This is only crucial for state schools, private (independent) schools will sometimes take out of year. But this is something to negotiate on.
Are your school fees going to be paid? Otherwise they tend to be about £18000, and a bit more at ACS. Fees can be assumed to rise 5% per year, in addition to any increases due to progressing through stages (eg. Junior to Senior).
How long are you planning to be in the UK? If long term, is there any specific reasons you would want them to go to a US rather than UK or European University?
ACS is a great school, especially if you are not going to be here long term, and are likely to live more in an expat "bubble". They also offer a excellent school bus service from places like Guildford and Woking (including a door to door service).
ISL is much newer.
Lots of overseas pupils do manage to get into the "usual" private schools, including the highly selective ones. I have known a US family who were here for a few years and all their children went to RGS and GHS in Guildford, which are top selective day schools.
On the other hand lots of expats put their children through the excellent State schools. You can always "appeal" to get a place at your preferred school, although places do occur even at the most popular schools.