So for example, India has this government organization called The National Council of Educational Research and Training (NCERT). They produce their textbooks which are also available to download for free here, for the grades 1-12 for all subjects. From my experience they are one of the highest quality textbooks in India.
Similarly, where can I access such textbooks which are of the highest quality, and are used in the US and/or UK? I'm mostly interested in searching for textbooks for the grades 8-12. I want to get a flavor of what kind of education do students get exposed to in the US/UK and I plan on reading their textbooks for the grades 8-12.