Safari has a built in feature called reading list, for a few years I used it as an alternative to bookmarks for articles. When I was looking at storage usage I saw that safari has 200mb+ for offline reading list, therefore the pages must have been chaeched. However, when I turn my phone into airplane mode and try to access any of the pages from reading list it prompts me an error 'The operation couldn;t be completed. Operation not permitted' I tried to search the web for troubleshooting the reading list and the only suggestion I found was to make sure that enable downloading for reading list using cellular is on and it was on. I am not sure what I can do in order to fix it.
On my iPhone 6 I need to turn off wifi and cellular data to read my reading list offline. If I have an Internet connection it will try to load the page from the Internet connection instead of the cached page.
Keep the tab open for a few moments. For larger sites, you may need to keep the tab open for a few moments to ensure that Safari saves everything. As long as the website isn't loading anymore, you're safe to close it.