Some of the books recommended are very weighty adult ‘classics’ that I would think are suitable for older readers. Not necessarily classics but adult books I read at that age included Sherlock Holmes, Agatha Christie, James Herriot, Gerald Durrell (started with My Family and Other Animals, then read obsessively), PG Wodehouse, John Wyndham and the Rumpole books. 39 Steps is a fab romp, very of its time.
Terry Pratchett wasn’t around then but I’d have to add to the list - but skip the earliest books and go in with the Guards or the Witches.
‘Proper classics’ - the only other Dickens I could manage then (or later) apart from Oliver Twist was A Christmas Carol, Treasure Island is great, To Kill a Mockingbird I read slightly older but loved. Kim by Rudyard Kipling.
Classic children’s books - Tom’s Midnight Garden, E Nesbit (Story of the Treasure Seekers has a nice ‘unreliable narrator’), Tom Sawyer & Huckleberry Finn (the latter is an amazing book though I dislike the final section), Just William series, Borrowers, Diana Wynne Jones, Swallows and Amazons.