As someone who deals with a great number of schools, from state comps to independents to grammar, I’m fairly confident you’ll find a wide and varied range of religious texts in most school libraries.
Seems reasonable as long as the schools provide text that dispute the religious texts as well – you know, the whole picture – there is, there isn’t, and all the varieties of each.
Does this include other external sources, such as the British Library, the British Science Association, the Historical Association, etc…?
Yep, as long as the motivation is known and up front. For example British Science Association are, I assume, promoting the scientific method. Which, on balance perhaps answers the first point. As long as my 2 are provided with the information, they are sufficiently intelligent to determine for themselves which route/s to follow.
Free books are great, as long as they are free in all senses of the word.