Which school in UK? (Istanbul)

Hello everyone!

May be I am thinking way too ahead for my little one :) but I was wondering which schools in UK are well known and recommended for 11+/13+/16+ year old kids? I studied in England as well but I did a PhD there so this doesn't help very much. I searched online as well but too many schools, too many options :/ If you want to send your kid to a school in UK (day time or boarding), how do you do that? Through agencies in Istanbul or by contacting the schools directly?

Thanks very much in advance!

Merry Xmas and happy new year everyone :)