Where can I find standard high school textbooks used in the US and UK?
In the US, at least, some of the states have lists of approved books. Texas for example has a list: https://repositories.lib.utexas.edu/handle/2152/19095
Maryland, or at least some large counties there, may also have such an approval process.
There are probably others. But note that some of the textbook selections in Texas are very controversial, as the selection board is sometimes dominated by creationists and other science deniers. So, use caution with this list and any other.